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Nitrido Complexes of Transition Metals.

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Nitrido Complexes of Transition Metals
By Kurt Dehnicke," and Joachim Strahle
In the past decade new syntheses and numerous structural determinations have enlivened
studies in the still relatively young field of nitrido-transition-metal complexes. Aside from the
terminal function of the nitrido ligand MEN:, this group also occurs as linear p,-bridging
ligand in symmetric and asymmetric coordination; examples are known with almost rightangled bridge function; and, finally, it also functions as p3-bridging ligand. Accordingly, the
fresh impulses given to synthetic chemistry by nitrido complexes are also many-sided: such
complexes are used, inter aha, for the preparation of phosphaniminato and thionitrosyl complexes as well as for the synthesis of metallaheterocycles of the type MN,S, and MN,P2 with
delocalized x-systems. In technetium chemistry complexes with terminal nitrido group are
employed as radiopharmaceuticals, and, owing to the strong trans influence of the M = N :
group, nitrido complexes of molybdenum are suitable as catalysts in olefin metathesis. Finally,
nitrido complexes are also of wide interest in theoretical studies.
1. Introduction
The following article surveys the development of the
chemistry of nitrido complexes containing transition metalnitrogen multiple bonds. It constitutes an updated continuation of our previous review article published in 1981.I1]Up to
that time only complexes with the terminal nitrido ligand A
and complexes with linear asymmetric and symmetric
bridges B and D, respectively, were known, while in the
meantime complexes containing bent bridges E with bond
angles as small as 91.3" have been synthesized, and in many
cases the unusual T-shaped bridge arrangement Fin trinuclear complexes with metal-metal bonds could also be prepared. Scheme 1 gives an overview of the hitherto established bonding functions of the nitrido-ligand.
= N+M
2. New Synthetic Strategies
2.1. Reactions with the Bromide of Millon's Base
Dominant among the new methods for the synthesis of
nitrido-transition-metal complexes are the reactions of metal
halides with nitride transfer reagents, which represent ligand
exchange reactions. One such reagent is Millon's base,
[Hg,N]OH, which is readily accessible in anhydrous form,[']
and despite its polymeric structure and consequent sparing
solubility is astonishingly reactive. For example, the bromide
of this base reacts with tungsten hexachloride in boiling carbon tetrachloride with formation of the binuclear p-nitridocomplex [W,NCI,]['] [Eq. (I), Scheme 2 (top)], which upon
heating furnishes WNCI, ,[11 a compound also accessible via
other routes [Eq. (2)].
2 WCI,
+ [Hg,N]Br
+ HgCI, + HgClBr
+ WCl,
Scheme 1. Bond functions of the nitrido ligand demonstrated so far.
A = terminal, B = asymmetric. linear bridge of the donor-acceptor type,
C = asymmetric linear bridge with covalent bond, D = symmetnc, linear
bridge. E = bent bridge, F = T-shaped arrangement.
Nitrido-complexes have also been treated in detail by Nugent and Mayer['] in a monograph on Metal-Ligand Multi[*I
ple Bonds. The present article does not deal with nitridocomplexes with interstitial N-atoms, as met with, e.g., in
or [(Ph,PAu),N]* .[41
Prof. Dr. K. Dehnicke
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-W-3550 MarburgiLahn (FRG)
Prof. Dr. J. Striihle
Institut fur Anorganische Chemie der Universitit
Auf der Morgenstelle 18. D-W-7400 Tiibingen 1 (FRG)
Angew Chum. lni. Ed. Engi. 1992,32, 955 -978
The halide nitrides can be separated from the accompanying mercury halides by heating in vacuo. In this way, starting
from the metal pentachlorides, the chloride nitrides of
molybdenum(v) and tungsten(v) were also obtained for the
first time [Eq. (3), M = Mo, W].r8x
+ [Hg,N]Br
+ HgCl, + HgClBr
This reaction can also be extended to the synthesis of
bromide nitrides. Thus, on heating tungsten hexabromide
with [Hg,N]Br in dibromomethane, WNBr, is smoothly obtained,"" from tungsten pentabromide the corresponding
[W2NBr,],["] (Scheme 2, below) and from molybdenum
tetrabromide the nitrido complex of tetravalent molyb-
Verlagsgesellsrhuji mbH, W-6940 Weinheim, 1992
+ .25,dJ
denum"" [Eq. (4)]. All these reactions proceed almost quantitatively and are thus also suitable for the synthesis of lSNlabeled complexes.[71
+ [Hg,N]Br
+ ZHgBr,
According to the TR spectra and magnetic susceptibilities
the halide nitrides are coupled both via nitrido- as well as via
halogeno-bridges, whereby high M-N bond orders are realized even in the case of compounds with bent M-N-M
bridges (Scheme 2).
Scheme 2. Schematic structure of the compounds [{W,NCI,J,] (top) and
[{W,NBr,),] (bottom).
The complex [Mo,NCI,], obtainable by reaction (3), can
be oxidized with chlorine in POCI, to give the mixed valent
MoV/MoV'chloride nitride [Mo,NCl,], which can be converted by reaction with tetraphenylphosphonium chloride
into (PP~,),[(MO,NC~,},].["~The structure of the anion
Fig. 1. Structure of the anion [{Mo,NCI,),]'-.
The white spheres are CI atoms
(Fig. 1) compares with that of the [{W,NCl,),] molecule,
however the Mo-N-Mo bridge with Mo-N distances of 167
and 212 pm is markedly asymmetric.["] The anion can
therefore be regarded as a donor-acceptor complex of
[MoNCl,]- and MoCl, . Indeed, it can be readily prepared
from these components in dichloromethane.['
Also [W,NCl,] can be oxidized with chlorine to the mixed
valent complex m,NCl,], which, unlike [Mo,NCI,] reacts
with PPh,CI with formation of the dianion [W2NCllo]2-.191
A crystal structure analysis of the AsPhl salt['21 revealed
eclipsed arrangement of the equatorial chlorine atoms and a
linear W-N-W axis with W-N distances of 171 and 203 pm,
consistent with the formulation W=N@-W@. Unclear is
which of the two tungsten atoms is pentavalent; possibly it
is the one with the short W-N bond, since with d' configuration one has to reckon with Jahn-Teller stabilization which
can be effective along the fourfold axis. Consistent with this
is the conspicuously short W-W distance of 373.3 pm, which
is significantly shorter than in W-N-W bridged tungsten(v1)
complexes such as [WNCl, POCl,], . 2POCl,['] with W-W
distances of 386.0pm. A W-W distance of 372.8 pm
analogous to that in the [W,NCI,J- ion was also found in
the tetrameric anion of (NHMe3),[{W0,C1,(H,0)),1 .
2H,O in which tungsten(v) and tungsten(v1) atoms are likewise coupled via linear W-0-W bridges.['31
Kurt Dehnicke was born in 1931 in Koln. He studied chemistry from 1950 to 1955 in Leipzig and
was awardedhis doctorate in 1957 at Stuttgart for research on boron trifluoride adducts of amino
acids under the supervision of J. Goubeau. In 1965 he habilitated in inorganic chemistry with work
on the chemistry of compounds containing electropositive chlorine. Two years later he accepted
a position as professor at the University of Marburg. In 1989 he was awarded the Wilhelm KIemm
Prize of the German Chemical Society. His main areas of research are transition-metal complexes with metal-nitrogen multiple bonds, alkyne complexes of transition metals in high oxidation
states, and polyselenido and polytellurido complexes.
Joachim Strahle, born 1937 in Dresden, studied chemistry in Stuttgart from 1958 to 1963, and
was awarded his doctorate in 1965 for research on metal azide halides and metal nitride halides
under the supervision of K. Dehnicke. After three years with G. Brauer in Freiburg andJive years
with H. Barnighausen in Karlsruhe (introduction to crystal structure analysis) he habilitated in
1973 with a treatise on the synthesis and crystal structures of metal-nitrogen compounds. In the
same year he took up an appointment at the University of Tiibingen as Head of the Department
of Inorganic Structural Chemistry. As successor to u! Riidorff (1975) he has occupied the chair
for Inorganic Chemistry I at Tiibingen since 1976. The areas of research carried out by his group
concerned the synthesis and crystal structure analysis of mixed transition metal gold clusters,
coordination compounds with nitrogen donor ligands, and metaljhoride nitrides.
Angew. Chem. l n t . Ed. Engl. 1992, 31. 955-978
2.2. Reactions with N(SiMe,), and with N(SnMe,),
Silylated and stannylated amines, even though infrequently used, are very efficient nitride transfer reagents.
Tris(trimethylsily1)amine reacts with tungsten hexachloride
in the cold to give very pure [W,NCI,] [Eq. (5)], at room
temperature to give WNC1,['41 [Eq. (6)].
+ N(SiMe,),
Fig. 3. Structure of the anion [W,NCI,J
+ 3CISiMe,
+ N(SiMe,), 5WNCl, + 3 CISiMe,
There are indications that the reactions ( 5 ) and (6) proceed
with stepwise cleavage of trimethylchlorosilane. In the case
of [MoNCIJ the reaction stops at the first substitution
step. The reaction product [MoNCl,{N(SiMe,),}]- can be
isolated as the tetraphenylphosphonium salt" 51 (Fig. 2). Be-
Fig. 2. Structure of the anion [MoNCI,(N(SiMe,),)]~.The white spheres attached to the M o atom are CI atoms, the spheres with central dots and attached
to the Si atoms are C atoms of the methyl groups.
rangement of the equatorial chlorine atoms is observed['41
(Fig. 3).
Other than in the [W,NCI,,]'- ion the two tungsten atoms
are now coupled via an almost symmetrical nitrido bridge
with a W-N-W bond angle of 173" and W-N distances of
179 and 188 pm, corresponding approximately to double
bonds. The marked trans influence of the nitrido ligand,
expressed in the 9 pm longer W-CI distances in comparison
to the usual W-CI bond lengths in cis position. Similar conditions are found in the anion of (PMe,Ph),[Ta,0C1,,],['71
which is isoelectronic with the [W,NCI,,]- ion. Here the
Ta-Cl distances Ta-CI,,/Ta-CI,,
(ax = axial, eq = equatorial) differ by only 4 pm because of the smaller trans influence of the 0x0 ligand of the Ta-0-Ta bridge.["]
In dichloromethane solution the [W,NCI,,]- ion slowly
decomposes with cleavage of tungsten hexachloride and formation of the trinuclear p-nitrido complex [W3NzC114]2-[1s1
[Eq. (711.
sides the very short Mo-N distance of the nitrido ligand
(164.8 pm), which corresponds to a Mo-N triple bond, the
Mo-N bond of the amido ligand (193.7 pm) also has a remarkable amount of rr-bond character. Accordingly the geometry around the nitrogen atom N1 is planar (Scheme 3).
Scheme 3. Planar environment at the N atom of the amido ligand due to partial
x-bonding of the Mo-N bond.
The metal-nitrogen bond has even more rr-bond character
in the monomeric nitreno-complex[*' [C13VG=N@-SiMe3]
(obtainable via an alternative route from VCI, and
Me,SiN,) with a V-N-Si bond angle of 177.5" and very
short V-N bond length of 159 pm. corresponding to a V-N
triple bond.['61
The [W,NCI,] complex accessible via reaction ( 5 ) reacts
with tetraphenylphosphonium chloride with formation of
[W,NCI,,]-, in which, as in ~ 2 N C I , , ] 2 - ~ " .eclipsed
L 2 1 ar-
In this article, complexes with linear M-N-X arrangement (sp-hybridized
N atom) are designated as niti-eno complexes, those with bent M=N-X
arrangement (sp'-hybridired N atom) as imido complexes [l].
Angru Clicm Inr. Ed. Enpl. 1992, 31, 955-978
The white spheres are C1 atoms.
+ WCI,
The structure analysis shows an almost linear arrangement of the skeletal atoms and eclipsed position of all twelve
equatorial chlorine atoms (Fig. 4). The W-N distances at the
Fig. 4. Structure of the anionic p-nitrido complex [W,N,CI,,]*-.
spheres are CI atoms.
The white
central tungsten atom W2 (207 pm) compare very
closely with the value expected for single bonds, whereas the
other two W-N distances (184 pm) can be regarded almost
as double bonds. The reason for the relatively long W-N
bonds at the central tungsten atom might be due to the
competition of the two N atoms for the electrons of the d,,
If one of the chlorine atoms in PN,NCI,] is substituted by
the bidentate chelate ligand of the dichlorophosphate ion an
unusual bridging of the W-N-W axis with simultaneous narrowing of the W-N-W bond angle to 166.2"[19] occurs
(Fig. 5).
formation of a bromide nitride [Nb,Br,N,] of unknown
To overcome this problem a new synthetic strategy was developed in which the ammonium halide
itself is used as reactant. In this way it was possible to synthesize the binuclear pnitrido-complexes (NH4),[Nb,NBrl0]
IEq. (9)1>( N H , ) ~ [ T ~ ~ N Iand
I O I (NH4)3[WzNBr~ollEq. (10>1
by reaction with the pentahalides of niobium and tantalum
as well as with tungsten hexabromide at temperatures between 300 and 420 0C.r241
Fig. 5. Structure of the anion [Cl,0WNWOCl,(0,PCI,)]2-. The white spheres
are CI atoms, the crossed spheres 0 atoms, the black sphere a P atom.
An even smaller M-N-M bond angle of 128" is found in
the structure of the cyclic complex [(Cp*TaN(Cl)),]
(Cp* = C,Me,) forming an almost planar six-membered
Ta,N, ring, and which can be regarded to a first approximation as triazatritantalabenzene[201 (Fig. 6). The Ta-N
+ 4NH4Br
6WBr, +14NH,Br
+ 4HBr
+ 20HBr + N,
According to the crystal structure analysis of
(NH,),[Nb,NBr,,] the anion has D,, symmetry and a linear,
symmetrical nitrido bridge with N b N distances of
184.5 pm, as would be expected for double bonds,[241
Roesky et al. overcame the problems of the amrnonolysis
of metal halides in another way when they treated the
trimethyl derivative [Cp*Ti(CH,),] with ammonia, and were
able to prepare a titanium imide nitride in good yields according to Equation (1 1) via elimination of methane.[2s1
Fig. 6. Structure of the benzene-analogous nitrido complex [{Cp*TaN(CI)},].
For clarity, the methyl groups attached to the Cp* ligand have been omitted.
The C atoms are depicted as small white spheres, the C1 atoms as large white
According to the structure analysis the nitrido-ligand N1
links all three titanium atoms, while the imido ligands form
p2-bridges between the Ti-atoms[2s1(Fig. 7). The Ti-N1 distances of 191 pm indicate a sniall amount of rr-bonding character.
distances vary only very little in the range from 184 to
191 pm, corresponding approximately to Ta=N double
bonds, if one regards the value of 185 pm in the anion of
(NH4),[Br,Ta=N=TaBr,]['1 as reference.
The compound was obtained in the form of shiny yellow
crystals upon reaction of [Cp*TaC14] and N(SnMe,), at
50 "C in toluene[201 [Eq. (8)J. The corresponding methyl
compound [{Cp*TaN(CH,)} 3]is formed upon ammonolysis
of [Cp*TaMe4][211[see Eq. (12)].
+ 3N(SnMe3),
+ 9Me3SnC1
The unusual stability of benzene-analogous cyclic trimers
of metal nitrides had been predicted shortly beforehand by
R. Hoffmann et a1.t221(see Section 6).
Fig. 7. Structure of the imidonitrido complex [{Cp*Ti(NH)},N]. For clarity
the methyl groups attached to the Cp* ligand and the H atoms at the imido
groups have been omitted.
The previously mentioned [{ Cp*TaN(CH,)},] could be
obtained in a similar way from [Cp*Ta(CH,),] and excess
ammonia in benzene at 20°Cr2'] [Eq. (12)].
2.3. Reactions with Ammonia
[CP*T~(CH,)~] NH,
Reactions of metal halides with ammonia generally lead to
mixtures of substances owing to the formation of ammonium halide as by-product. Thus, for example, ammonolysis of
niobium pentabromide leads to a mixture of amido- and
imido-derivatives and NH,Br, and, upon heating at 220 "C
simultaneous sublimation of the ammonium bromide and
+ 3CH4
The binuclear p-nitridoosmium(1v) complex [ (NH,),OsNOs(NH,),]CI, . H,O is formed in astonishingly good yields
in a strongly exothermic reaction upon treatment of solid
(NH,),[OsCl,] with hydrazine hydrate.IZ6]The reaction inAngew. Chrm. Int. Ed. Engl. 1992, 31, 955-978
volves disproportionation of hydrazine into N, and NH,, so
that it can be formulated as in Equation (13).
coupling motif of cubic TaN, into which the compound can
be converted by heating at 820 0C.[28]
For further details on the T-shaped coupling of metal
atoms and on nitrido bridges with very small bond angles,
see also Section 3.4.
+ 14 N H 3 + H,O
+ 7NH,CI
Owing to the strong wuns effect of the nitrido ligand of
the linear 0s-N-0s bridge, the truns NH,-ligands can be
readily replaced, so the complex can serve as starting
material for the synthesis of complexes of the type
[X(NH,),0sNOs(NH,),X]C15 .[261 Interestingly, reaction
(13) takes another course if the components are allowed to
react with each other in the reversed order. Addition of solid
(NH,),[OsCl,] to hydrazine hydrate then leads to formation
of the dinitrogen complexes [Os(NH,),(N,)]CI,
[Os(NH3),(N,),IC1, .[261
A similar result as depicted in Equation (13) is observed
however in the albeit experimentally more time-consuming
reaction of (NH,),[OsCI,] with concentrated aqueous ammonia at 150 "C in a Carius tube. After 9 days the p-nitrido
complex [Os2N(NH,)8CI,]CI, . 2H,O was obtained.[,']
The result of the reaction of (rBuCH,),Ta=CHtBu) with
ammonia at room temperature in benzener2'1 [Eq. (14)] is
(rBuCH,),Ta=CHfBu 2 ~[{(fBuCH,),TaN},] + 2CMe,
2.4. Reactions with Nitrogen Trichloride
Reactions with nitrogen trichloride do not present any
danger when carried out in CCI, solution; their use for the
synthesis of nitrido complexes is, however, somewhat limited, but in the few cases where they can be used they are very
efficient. Aside from the previously described synthesis of
[ReNCl,] from ReCI,['] and the reaction of MoC1, with
NCI,, which led to [MoNCI,], ,[*'I none of the known metal
chlorides was found to react with NC1,.
In contrast, reactions of the hexacarbonyls of molybdenum and tungsten with excess nitrogen trichloride led in
almost quantitative yields to the synthetically very useful
N-chloronitreno complexes [Eq. (15), M = Mo, W].[30,311
The N-bound chlorine atom in these compounds has a
marked (6 +)-character. Thermolysis accordingly leads with
cleavage of chlorine to the chloride nitrides MNCI, [30, 3 1 1
[Eq. (1611.
most remarkable. The product crystallizes from toluene together with one coordinated NH, molecule and with the
composition [{(tBuCH,),TaN},] . NH, . 2C,H,; a crystal
structure analysis indicated the unusual T-shaped coupling
of three of the five nitride groups and two p,-nitrido bridges
with a very small bond angle of 9Sor2*1(Fig. 8).
+ 4NC13
+ N, + C1, +12CO
2MNC13 + 2C1,
To our knowledge this is the most efficient entry to the
chloride nitrides MoNCI, and WNCI, ,which in turn are the
most important starting substances in the complex chemistry
of the nitrides of molybdenum and tungsten.
The partial positive character of the N-bound CI atom is
also manifested in the capability of the pentachloro derivative [Cl,WeN-CI]- as PPht salt to bind a chloride
(Fig. 9). The chloride ion coordinates with formation of a
linear C1,-bridge between two anions.
Fig. 8. Structure of [{(rBuCH,),TaN},] ' NH,. The white spheres are C atoms.
The Ta,N, skeleton is almost planar; it is best formulated
as in Scheme 4. The edge-sharing Ta2N, units reproduce the
Fig. 9. Structure of the binuclear N-chloronitrene complex with the bridging
chloride ion [{Cl,W(NCl)),Cl1'-.
2.5. Oxidation of Coordinated NH,-Ligands
Scheme 4. Schematic
Angen.. Chem. I n f . Ed. Engl. 1992, 31. 955-978
An elegant entry to nitrido complexes of osmium and
chromium was opened up by the reaction of strong oxidizing
agents such as c e r i u m ( ~ v )[Eq.
[ ~ ~(17)]
~ or sodium hypochloriteC3,][Eq. (1811 with complexes containing NH, ligands.
Reaction (17) corresponds to an oxidation of the osmium
atom from $ 1 1 1 to +VI, reaction (18) to an oxidation of
the chromium atom from + I I I to t v . In exactly the same
way it was also possible to synthesize the first nitrido complexes of manganese(v), namely [MnN(ttp)] and [MnN(oepMe,)] [ttp = 5,10,15,20-tetra-p-tolylporphyrinate(2- ) ;
(oepMe,), - = a-y-dimethyl-x-y-dihydrooctaethylporphyr i r ~ a t e ] . [361~ ~ %
[ O S ( N H , ) , ] ~ ~ 3Ce4+
+ H,O
+ OCI-
+ 3Ce 3+ + 2H' + NH:
+ C1- + 2H,O
According to crystal structure analyses the metal atoms in
the porphyrinato complexes are fivefold coordinated, with
extremely short M = N : distances of 156.5 pm in the case of
the chromium complex[371and 151.2 pm in the case of the
manganese complex.[361Upon chlorination of the NH, ligand in the phthalocyaninatomanganesecomplex [ (NHJMn(OH)pc] with elemental chlorine, which leads according to
Equation (1 9) to the corresponding nitridomanganese(v)
complex, intermediary formation of the N-chloronitreno
complex [(CIN)Mn(pc)] with manganese(rv) is assumed.[381
+ 2'21,
2[:N~Mn(pc)] 2 H 2 0
+ 4HCl
proceeds analogously; however, in this case, formation of
the linearly bridged, trinuclear complex [{MoN(N,)(depe),},MoN(N,),] with a bond angle of 156.3" at the central molybdenum(v) atom is also observed (Scheme 5).[421
The terminal Mo=N: bond (167 pm), as expected, is somewhat shorter than the Mo''=N bridge-bond length (170 pm).
Cyclovoltam~netricmeasurements show a reversible oneelectron reduction of the molybdenum(v) atom with a potential (EO')of -2.10 V relative to ferrocenium/ferrocene in
T H F solution.[421
Thermolysis of the tetraazidepyridine complex of molybdenum leads to the corresponding monomeric triazidonitrido complex with fivefold coordinated molybdenum atom[431
[Eq. (20)], whereas reaction of the tetrachloroterpyridyl
complex with trimethylsilyl azide leads, after partial
Cl/N, exchange, to the azido-chloro-nitrido complex
[Eq. (21)]. In this complex, the
+ 3Me3SiN3
+ N, + 3C1SiMe3
molybdenum has an unusual pentagonal-bipyramidal environment with the chloro ligand in the axial position[441
(Fig. 10).
2.6. Thermolysis and Photolysis of Azido Complexes
The chemistry of coordinated azide groups was reviewed
in 1985.[391Many of the known decomposition reactions of
azides, which lead to nitrido complexes, are thermally
induced.['.391The conversion of an azido ligand M-N,
into a nitrido ligand M E N : via N,-cleavage is accompanied
by a two-step oxidation of the metal atom. The formation
of the nitrido complex [M~N(N,)(dppe),]["~](dppe =
observed in the reaction of
[Mo(N,),(dppe),] with trimethylsilyl azide would therefore
suggest [Mo(N,),(dppe),] as being the initial product. In the
nitrido complex the Mo-N distance to the nitrido ligand
(179 pm) is remarkably long; it corresponds almost to a
double bond.[401
In the synthesis of the corresponding tungsten complex
wN(N,)(dppe),], the authors found evidence of a radical reaction following previous replacement of N, by
Me,SiN, .I4']
The synthesis of trans-[MoN(N,)(depe),]
(depe =
Et,PCH,CH,PEt,) starting from trans-[Mo(N,),(depe),]
\ /E N -Mo i-N p\3 M /po - N,
N3 -Mo
Scheme 5. Schematic representation of the linearly bridged, trinuciear complex
[iM W N d d e p e ) , ) ,MoN(N,),I.
Fig. 10. Structure of the nitrido complex [MoN(N,),Cl(terpy)]. With exception of the atom labeled as CI, all white spheres are C atoms.
The nitridoniobium(v)-complex [(NbNCI, . SMe,),]
could be prepared in a corresponding way by reaction
of trimethylsilyl azide with the niobium(II1) complex
[Nb,C1,(SMe,),].[451 Osmium(1v) complexes can also be
readily oxidized with trimethylsilyl azide to give nitridoosmates(vI), as shown by the example in Equation (22)
(L = PPh,).[46'
The origin of the [Ph,PNH,]+ ion can be explained by
hydrolysis of the SiMe,NPPh, formed at the same time by
Staudinger reaction.
Addition of (NEt,H,)[S,P(OMe),] to a mixture of [MoCl,(thf),] and trimethylsilyl azide leads to formation of orange-brown crystals of the tetrameric complex [{ Mo'N[S,P(OMe),]2}4], the sole example of a square Mo,N, unit with
four symmetric Mo=N=Mo bridges.["'] The Mo-N bond
distances are, within statistical deviation, equal and with a
value of 186.5 pm correspond very well with the value exAngen'. Chem. Int. Ed. Engl. 1992, 31, 955-978
130 C
+ 2SC1,
PPh,[MoNCI,] + SCI,
+ AsPh,CI
20 “C
+ POCI, + SCI,
Reduction of the chlorothionitreno complex used as educt
in Equation (27) with triphenylphosphane leads in good
yields to a nitridorhenium(v) complex[551[Eq. (28)].
+ 9PPh,
pected for double bonds. Consistent with this finding, the
observed ferromagnetism is explained in terms of a super’]
exchange mechani~m.’~
The photolytic decomposition of a chromium azide was
first described in 1981.[481 Irradiation of azido-NJ”’
ethylenebis(salicylaIdiminato)chromium(~~I)with light of
wavelength 320 nm smoothly furnished a nitrido chromium(v) complex[481[Eq. (23)].
[Cr(N,)(salen)] . 2H,O
2 [CrN(salen)]
+ N, + H,O
Later, Groves et al. obtained by photolysis of the azidotetra-p-tolylporphyrinatochromium(m)complex in CH,CI,
solution the corresponding nitrido complex of pentavalent
chromium in which, according to the crystal structure analysis, the chromium is fivefold coordinated and the nitrido
ligand is in the apical position.[371In an analogous way azidometal(1n)porphyrins of chromium, manganese and iron
were also converted into the corresponding nitrido complexes [MN(ttp)],[“’] and from [Cr(N,)(bpb)] [bpb = 1,2-bis[2pyridinecarboxamido(2-)]benzene] the nitrido complex
[CrN(bpb)] was obtained.[”]
2.7. Reactions with Trithiazyl Chloride
Some nitrido complexes are best prepared by reaction of
metal chlorides with the readily accessible trithiazyl chloride
(NSCI), . Initially, chlorothionitreno complexes are formed
in which the [N=S-C1J2- ligand, which does not exist in the
free state, is bound at the metal atom with linear arrange0
ment of the elements M-N-S as M=R=S-Cl.[511 An example is provided by the reaction with tungsten hexachlo[Eq. (24)].
Some of the initially formed chloronitreno complexes furnish the corresponding nitrido complexes under gentle, thermally-induced conditions [Eq. (25-27)].[53. 541
Angcw. Chern. Int. Ed. Engl. 1992, 31,955-978
+ 3PPh3CI, + 2PPh3S + 2POC1,
This complex had already been prepared earlier by Chatt
et al. by reaction of Na[ReO,] with hydrazine dihydrochloride in the presence of PPh,;r561according to the structure
the rhenium atom is fivefold coordinated with a
coordination geometry between trigonal-bipyramidal and
tetragonal-pyramidal. In contrast, the corresponding nitrido
complex with the less bulky dimethylphenylphosphane,
[ReNCl,(PMe,Ph),], contains sixfold coordinated rhenium
with one of the chloro ligands trans to the nitrido
In attempts at the transhalogenation of the chlorothionitreno complexes of molybdenum and tungsten with
trimethylbromosilane only [WCI,(NSCI)] could be converted into the bromo complex [WBrJNSBr)], whereas the
molybdenum compound reacted smoothly and quantitatively with formation of MoNB~,[~’][Eq. (29)]. Reaction (29)
opens up a simple preparative entry to MoNBr, compared to
the synthesis from MoBr, and iodine azide.“]
+ IOBrSiMe,
+ Br,
The transhalogenation of [{Cl,Mo(NSCI)},J with sodium
fluoride in acetonitrile solution in the presence of [15]crown5 as phase transfer reagent at 80°C also led to a nitrido
complex[601[Eq. (30)].
+ 12NaF + 2[15]crown-5
[Na([15]crown-5)],[(MoNF4),1 + 2SF2 lONaCl
The compound crystallizes as ion triplet (Fig. l l ) , in
which the sodium atoms coordinated fivefold by the crown
ether molecules form short Na . .. F contacts to the axially
arranged F atoms of the [(MONF,),]~- ion.
The Na . . . F distances of 234 pm are only slightly longer
than in the sodium fluoride structure (230.7 pm). The molybdenum atoms of the anion are coupled via asymmetric fluoro
bridges with Mo-F distances of 198 and 245 pm, whereby
the long Mo-F bond is a result of the strong trans influence
of the terminal nitrido ligands. The small Mo-F-Mo bridge
angle of 109”is unusual, since up to now only bridging angles
96 1
Fig. 11. Structure of the ion triple [Na([15]crown-5)],[(MoNF4),]. The white
spheres are F atoms, the smaller white spheres C atoms, and the crossed white
spheres 0 atoms.
between 132" and 180" have been observed for fluorides of
4d and 5d
2.8. Direct Syntheses from the Elements
Kniep et al. were able to prepare the ternary nitrido complexes Li,[MN,] (M = Cr, Mo, W)[621 and BaJMN,]
(M = Mo, W)[631 from the elements by high-temperature
syntheses; the complexes were obtained as pale yellow to
reddish-brown, moisture-sensitive single crystals. According
to the crystal structure analyses only slightly distorted
[MN,I6- tetrahedra are present, in which the bond lengths
(M = Cr, 176-178 pm; M = Mo, 185-189 pm; M = W,
189- 193 pm) have marked n-bond character.
Fig. 12. Structure of the complex [VNCl,(pyridine),J. The white spheres are C1
atoms, the smaller white spheres H atoms, and the spheres with central dot C
vanadium atom are formed. The V-N distance of 157 pm in
[VNCl,(quin),] corresponds to a triple bond.1651
The reaction of molybdenum(n) benzoate with trimethylsilyl azide in the presence of PPh,CI led in an unexpected redox reaction with participation of the solvent
dichloromethane to the azidochloronitrido complex
[ { M o N ( N , ) , C I } , ] ~ - ~[Eq.
~ ~ ~(3211.
+ 2PPh,CI + 4CH,CI, + 8Me,SiN,
(PPh,)2[{MoN(N,),Cl),1 + 4Me,SiOC(O)Ph + 2C2H,C1,
+ 4Me3SiC1+ 2N,
In contrast to the ponomeric structure of the tetraazidonitrido complex [MoN(N,),] -,[69, 701 the chloro derivative [MoN(N,),Cl]$ - is coupled via the a-N-atoms of two
azido groups to give a centrosymmetric dimer with Mo-N
distances of 208 and 260 pm[681(Fig. 13). The long Mo-N
2.9. Special Synthetic Methods
The [VNCl,]- ion, isoelectronic with the VOC1, molecule,
is obtained upon reaction of [Ph,PNPPh,]CI with [Cl,V=NSiMe,] in dichlor~methane[~~I
[Eq. (31)].
The V=N bond (6.9 Ncm-') has an equally large valence
force constant as the V=O bond in the VOCI, molecule.[641
+ CI,VNSiMe,
+ CISiMe,
Fig. 1 3 . Structure of the anion [{MoN(N,),CI},]2-.
Also the reactions of Cl,VNSiMe, with bases such as 2,2'bipyridyl (bpy), pyridine (py) or quinuclidine (quin) lead to
a cleavage of chlorotrimethylsilane and formation of the
complexes [VNCl,(bpy)],[641 [VNCl,(py),][651 and [VNCI,(quin),],
On the other hand, the trimethylsilylimido complexes
[MCl,(NSiMe,)L,] (M = Mo, W; L = phosphane) and
[WCl,(NSiMe,),L] show no tendency to be converted into
the corresponding nitrido complexes.[661The pyridine complex [VNCl,(py),] (Fig. 12) is also formed upon heating the
cyclothiazeno complex [{VCl(N,S,)(Py),},] in toluene.[671
According to crystal structure analyses the [VNCl,(py),]
complexes are stacked into columns via alternating long
VEN...VEN bridges with V-N distances of 160 and
256 pm.[653
With bulky amino bases such as quinuclidine, monomeric,
readily soluble complexes with fivefold coordination at the
distance is in the trans position to the terminal nitrido ligand,
whose M o r N distance of 164pm corresponds to a triple
A surprising redox reaction also forms the basis of a new
alternative method for the synthesis of the nitrido complex
[{MoNCl, . POCI,},], which was obtained by heating the
phosphoryl trichloride solvate [MoCI, . OPCI,] with urea in
boiling carbon tetra~hloride[~']
[Eq. (33)].
4[MoCI,. OPCI,]
+ 2OC(NH,),
[{MoNCI, OPCI,},]
+ 2CO + SHCl
Surprisingly, the elimination of PhS0,Cl from the nitreno
complex [CI,Mo=NSO,Ph] also takes place in reverse to the
Angew. Chem. In[. Ed. Dtgl. 1992, 31, 955-978
electrophilic addition of PhS0,CI to the nitrido function (cf.
Section 3.2) upon reaction with [Ph,PNPPh,]CI in
d i c h l o r ~ m e t h a n e ~[Eq.
’ ~ ~(34)].
+ 2[Ph,PNPPh3]Cl
+ PhS0,CI
An unusual entry to a nitridotungsten complex was found
upon attempting to prepare an amidinato complex from
tungsten hexachloride and N,N,N’-tris(trimethyIsily1)benzamidine[731[Eq. (35)].
+ [Ph-C(NSiMe,)N(SiMe,),]
[WNCl, . NC-Ph]
+ 3 CISiMe,
Fig. 14. Section of the polymeric structure of [(iBuO),W=N]. The white
spheres are C atoms, the crossed spheres 0 atoms. For clarity, the methyl
groups of the rBu residue have been omitted,
nitrenonitrido complex C P h l [Ph,C-N=WCI,(p-F)WNCl,(p-F)Cl,WrN-CPh,]- (Fig. 15), and in the structure of the
trinuclear molybdenum complex [(p-MoN(N,),} {NMo(N3)(depe)2}2]t4z1
already described in Section 2.6.
Whereas numerous metal halides react with the silylated
benzamidine to give amidinato complexes of type G,[741
reaction (35) does not stop at this stage but leads, with further cleavage of CISiMe, and formation of benzonitrile,
to the nitridotungsten complex, which crystallizes as the
tetramer.[731 The acetonitrile solvate [WNCl, . NCCH,],
which was obtained by reaction of tris(trimethylsily1)amine
with the N-chloronitrenotungsten complex, also crystallizes
as the t e t ~ - a m e r [[Eq.
~ ~ ] (36)].
3 fWCI,(NCI)(CH,CN)]
+ N(SiMe,),
+ $ N z +lfCI, + 3CISiMe,
Fig. 15. Structure of the anion [WNF,CI,(CI,WNCPh,),]~. The smaller white
spheres which bridge two W atoms are F atoms; the larger white spheres
attached to the W atoms are C1 atoms. The white spheres which are coordinated
to the N atoms are C atoms of the triphenylmethyl groups.
Reactions of tert-butyl isocyanate with the dioxo complexes [RuO,CI,]- and [MO,C1,IZ- (M = Ru, 0s) in the
form of their tetraphenylphosphonium salts in boiling acetonitrile led to some surprising results. The air-stable nitrido
complexes PPh,[MN(tBuNC(0)NlBu)C12] are formed
which contain a N,N’-ureato(2-)-chelate
With an original metathetic reaction of tert-butylcarbonitrile and [(tBuO),W=W(OtBu),] leading to cleavage of the
W r W bond, Schrock et al. were able to prepare at one and
the same time a nitrido complex and a carbyne complex of
t u n g s t e n ( v ~ ) [[Eq.
~ ~ ](37)].
+ [(tBuO),W=W(OtBu),]
+ [(~BuO),W=CIBU]
According to the crystal structure analysis the nitrido
complex contains W=N- W chains with markedly alternating W-N bond lengthsr7’] (Fig. 14). The corresponding
molybdenum compound has a comparable structure with
Mo-N distances of 166 and 286 pm.[781
The rare trigonal-bipyramidal coordination was previously only observed in the structure of the trinuclear
An,w:r,i Chrm In!. Ed. Engl. 1992, 31, 955-978
Fig. 16. Structure of the anion [OsN(tBuNCfO)NrBu)Cf,]~.The spheres with
central dot are C atoms, the crossed spheres 0 atoms.
(Fig. 16). The formation of the ureato ligand most likely
proceeds via an imido complex as intermediate [Eq. (38)].
t N C 0 Il/O\
2 M=N
The ureato complex can subsequently react with a further
molecule of tert-butyl isocyanate to give an imido complex,
from which the nitrido ligand is finally formed by elimination of 2-methylpropene and HCl[sO1[Eq. (39)].
3. Chemical Reactions of the Nitrido Complexes
Many of the chemical reactions dealt with here lead with
retention of the nitrido function to novel nitrido derivatives
so they are also to be grouped under the heading of new
synthetic routes. In other reactions the nitrido function itself
serves as reactive group, and enables the synthesis, in particular, of new metallaheterocycles.
3.1. Addition of Lewis Bases
The uncomplexed halide nitrides, such as MoNCI, , WNCl,, ReNCl,, MoNBr,, and WNBr,, despite their polymeric structures, have Lewis acidity. They react with various
Lewis bases with formation of well-defined donor-acceptor
complexes. Of the uncomplexed halide nitrides, the only
compound crystallographically characterized is MoNCI, ,
which forms [MoNCI,], units in the lattice; these are associated via chloro bridges MoC1,Mo to form a layer lattice.r811
In reactions with Lewis bases the formation of tetrameric
molecular structures [MNCl, . D], (D = donor molecule) is
preferred. Since all halide nitrides MNX, can form tetrameric units with Lewis bases, these compounds presumably have
the same structure as MoNCI, .
In the reaction of WNCI, with hydrogen azide as Lewis
base only every second tungsten atom coordinates with the
r-N-atom of HN,. This leads to only some of the chloro
bridges of WNCI, being ruptured, and a band structure results[821(Fig. 17).
Upon coordination with trichloroacrylonitrile a degradation of the polymeric structure of MoNCI, down to a dimeric
unit occurs, and terminal nitrido ligands result[s31(Fig. 18).
Fig. 17. The coupling of the tetrameric complex [{WNCI, . OSHN,},]
Fig. 18. Structure of the dimeric complex [{MoNCI, . NCC,CI,},]. The white
spheres are CI atoms, the spheres with central dot C atoms.
The Mo-CI distances of 240.7 and 276.0 pm in the
MoC1,Mo bridges are markedly different, the long Mo-Cl
distance being determined by the trans influence of the nitrido ligand.
Bifunctional donor solvents suitable for chelation, such as
dimethoxyethane, lead in the reaction with MoNCI, to
monomeric complexes which are readily soluble in organic
solvents (Fig. 19). The Mo-0 bond to the 0 atom, which is
Fig. 19. Structure of [MoNCI,(MeOC,H,OMe)~.The spheres with central dot
are C atoms. those that are crossed 0 atoms.
in the trans position to the nitrido ligand, is long (247 pm)
because of the trans-influence, whereas the Mo-0 bond
length of the cis 0-atom is 215 pm.[s41Polymers with attached MoNCI, units can be obtained if the polymer bears,
e.g., bipyridyl residues as functional groups[851(Scheme 6).
Reaction of the halide nitrides with anionic bases leads
finally to the known monomeric complex ions [MNX,]-[']
with C,, symmetry and with the M s N group in the axial
position. The absence of association of the [MNXJ ions is,
Angebv. Chem. Inr. Ed. Engl. 1992. 31, 955-978
on the one hand, a result of the repulsive negative charge, but
also a result of the frequently employed counterions PPh: or
AsPh: ,which because of their bulkiness counteract an association. However, if instead, one uses the [CI,PNPCI,]+ ion,
then also [MoNCIJ- ions can stack in columns[861(Fig. 20).
Fig. 21. Structure ofthe borylnitrene complex [Re(NBCl,)Cl,(PMe,Ph),]. The
white spheres which are coordinated to the B and Re atoms are CI atoms; the
white spheres coordinated to the black P atoms are C atoms.
Fig. 20. Section of the chain structure of [MoNCIJ
The long Mo-N bond (243 pm), however, corresponds to
only a very weak interaction.
Especially interesting is the acceptor behavior of the
binuclear nitridoporphyrinatoiron complex [Fe,N(tpp),]
(H,tpp = 5,10,15,20-tetraphenylporphine),which according
to the EPR spectrum has an A,-ground state with two equivalent iron centers of formal oxidation state Fe+3.5.r871
Addition of one mole of pyridine leads to non-equivalence of the
two iron atoms, indicating an FeIVcenter with coordinated
pyridine and coordination number six and an Fe"' center
with coordination number five. Equivalence of the two iron
centers occurs again only at high pyridine concentrations.[871
At low temperatures even an 0, molecule can add to
[Fe,N(tpp),],[''~ whereby, according to the 57Fe-Mossbauer
spectra, the two iron atoms become non-equivalent again.
The formation of the I : 1 adduct, which however can only be
described as a weak interaction,["] could be demonstrated
by an EPR spectrometric titration.
The electronic properties of the complex [Fe,N(tpp),] are
similar to those of complexes of the type [Fe,N(pc),]
(H,pc = phthal~cyanine)['~](see also Section 6). These
complexes can easily be oxidized with bromine, trifluoroperoxoacetic acid or nitric acid to the air-stable radical
cations [Fe,N(pc),]+ with Fe'"-center~.[~~~
Like the corresponding porphyrinato derivatives[901they serve as model
substances for the mode of action of hydroperoxidase enzyme.["I
3.2. Addition of Lewis Acids
The addition of Lewis acids to nitrido complexes has hitherto attracted relatively little interest. In all known examples
the addition takes place at the nitrido ligand. Already known
for some time are addition reactions of boron trihalides to
rhenium(v) complexes which lead to complexes of the type
(X = CI, Br; Y = F, CI, Br).[921
Later, the corresponding methylphosphane derivative
[CI,RerN- BCI,(PMe,Ph),] was crystallographically chara ~ t e r i z e d .According
to the crystallographic analysis the
structural group Re=N-BCl,, as in the analogous nitreno
Angex. C'l7rm. l n t . Ed. Engl. 1992, 31. 955-978
complexes M=N-R,['is linear with sp-hybridized N
atom (Fig.21).
The addition of the BCI, molecule causes a stretching of
the Re-N bond length from 166 pm in the nitrido complex
[ReNCI,(PMe,Ph),] with terminal Re=N grouprss1 to
173 pm.[931 At the same time the trans-influence of the
NBCI, ligand4ompared to the nitrido ligand-decreases
very strongly, as evidenced by the difference in the Re-CI
bond lengths between cis and trans Re-CI bonds, which is
only 4 pm in the boryl complex, whereas in the nitrido complex a difference of 20 pm was found.[581
A much smaller tendency for the addition of boron trihalides is observed in the case of the nitridorhenium(v1) complexes. Whereas BCI, adds neither to [ReNCI,]- nor to
[ReNBr,]-, with boron tribromide a stable adduct is obtained. It is formed upon transhalogenation of the chloro
complex with excess boron t r i b r ~ m i d e . ' ~[Eq.
~ ] (40)].
3AsPh4[ReNC1,] +7BBr3
+ 4BC1,
Above 200 "C the bromo complex loses BBr, leading to
formation of A S P ~ , [ R ~ N B ~ , ] . [ ~ ~ ]
Despite the weakening of the Re-N bond by the addition
of the boron trihalides, a shift of v(ReN) to shorter wavelengths is generally observed in the IR spectra. Thus, this
band changes its position from 1099 cm- ' in the spectrum of
AsPh,[ReNBr,] to 1170 cm-I after addition of BBr,.[941
The effect is due to a vibrational coupling with the B-N
stretching vibration, which in turn has a long-wave shift.
The electrophilic attack of the triphenylcarbonium ion on
the nitrido function of molybdenum, tungsten and rhenium
complexes leads to the synthesis of imido complexes
[Eq. (4111.
M = N : + Cph:
[M+J -Cph, ]+
First examples of such syntheses were found by Chatt
et al. on using as starting material the dithiocarbaminato
complexes [MoN(S,CNMe,),] and [ReN(S,CNMe,),(PMe,Ph)].[951The structure analysis of the molybdenum
complex [Mo(NCPh,)(S,CNMe,),][BF,] threw light on the
pentagonal-bipyramidal coordination at the molybdenum
atom with the MorN-CPh, group in the axial position.[951
Also other electrophilic reagents such as PhCOCI, PhSO,CI, ArSCI, ArCl and R,OfBF, react in a similar way at
the nitrido function[951(Ar = aryl, R = Me, Et).
Of importance is the activation of nitrogen and its transfer
to olefins in the reaction of nitrido-porphyrinato (por) complexes of manganese by reaction with trifluoroacetic anhydride. An electrophilic addition takes place at the nitride-nitrogen with formation of a 'nitrenoid'-manganese-porphyr i ~ ~ a t e[Eq.
] This can then react in a second step with
olefins to give aziridinesrg6I[Eq. (43)].
+ O(COCF,),
[(por)Mn-N-C(O)CF,] +[CF,COJ
+ 3CPh,BF4
+ 2BF, + [BF4]- + 2CI-
.. e
[CI,W = N-P(C1,)-N=PCl,]
Also this reaction can be regarded as electrophilic attack
of one of the P atoms of the cation at the nitrido ligand. The
atoms of the W-N-P-N-P chain of the phosphoraniminate
Fig. 22. Structure of the phosphoraniminato complex [Cl,WNP(Cl,)NPCl,].
The white spheres are CI atoms.
The phosphoraniminato ligand [P,N,Cl,]-, unknown in
the free state, is isoelectronic with the molecular phosphazene P,ONCI,, which also has a planar atomic skeleton
and comparable bonding
Addition of molybdenum pentachloride to the chloronitridomolybdates [MoNCI,]- and [MoNCI,]*- leads to the formation of p-nitrido-complexes. Whereas the complex
[Mo,NCl,]- with Mo" and MoV1 centers formed with
[MoNCl,]- contains a highly asymmetric Mo%N-Mo"
bridge (see Fig. I), the spectroscopic findings for the
[Mo,NCI,12 - ion accessible from [MoNC1,I2 - indicate presence of largely symmetric Mov=N=MoV bridges" Ool
(Scheme 7).
A surprising result was observed in the reaction of
[CI,PNPCI,]Cl with WNCI,, in which initially the expected
chloronitrido complex was formed; however, this slowly rearranges in a secondary reaction to a phosphoraniminato
[Eq. (45)].
Reaction (43) can be regarded as the aza-analogous epoxidation of olefins. Kinetic studies have shown that the previous reaction (42) depends very strongly on the basicity of the
porphyrinato group and on the steric conditions at the
Mn-N function.'971
More complicated is the reaction of CPhTBF; with the
chloronitridotungstate AsPh,fWNCl,], which is accompanied by a partial Cl/F exchange that leads to a trinuclear nitrenonitrido complex CPh: [Ph, C-N- WCI,( p-F)WNCI,(p-F)CI,W-=N-CPh,l- (Abb. 15) [GI. (44)], associated via the two F atoms.r791
are almost coplanar, and the P-N bonds, the lengths of
which differ only slightly, point to a through conjugation of
this chain[981(Fig. 22).
1 4-
Scheme 7. The Structure of [Mo,NCIJ
units associated to dimers
Similarly, structures associated to dimers are also found in
the case of the previously mentioned W,NCl,] (Section 2.2),
as also in the case of the [Nb,OCI,]- ion isoelectronic with
[MO,NCI,]~- .rloll
A nitrido bridge ReSN-Au is formed between two different metal atoms upon addition of the AuCl molecule accessible from [Au(CO)CI] to the nitrido function of
[C1,Re~N(PMe,Ph),]."02' As the crystal structure analysis
shows, the resulting p-nitrido complex [CI,Re=N-AuCI(PMe,Ph),] has a structure analogous to that of [Cl,ResNBCI,(PMe,Ph),][931 with the metallanitrene function in
trans-position to a CI ligand and meridional arrangement of
the phosphane ligands.['021Whereas the formation of this
complex is to be regarded as addition of AuCl to the nitrido
function, the complex [ (Me,SiO),VrN-Pt(Me)(PEt,),] prepared by Doherty et al.[lo3]by reaction of [(Me,SiO),V=NSiMe,] with [FPtMe(PEt,),], despite an analogous atomic
sequence in the bridge VGN-Pt (compared to ReGN-Au), is
a metallanitreno complex.
Angew. Chem. Int. Ed. EngI. 1992,31, 955-918
Also alkali-metal fluorides such as NaF or K F can be used
for the fluorination if the reactions are carried out in acetonitrile in the presence of suitable crown ethers. The fluoronitrido metalates are formed in which their cations form ion
pairs or triplets owing to the anisotropic coordination by the
crown ether molecules. An example is reaction (49), which
goes to completion very rapidly at room temperature.[' 07]
The remarkable bond relationship of the ,!l-N-atom of the
covalently bound azido ligand of [W(N,)CI,] to the nitride
function of WNCI, can also be regarded as addition of a
Lewis acid["41 [Eq. (46); here, both the formal charges as
well as the chlorine atoms are omitted].
+ NI
+ 5 K F + [18]crown-6
[K([18]crown-6)]D;VF5(NCI)] 4KCI
The positively charged CI atom on the nitreno ligand is not
exchanged (Fig. 24).
In the presence of tetraphenylarsonium chloride this reaction leads to the pisotetrazenido(4-) complex of tungsten(v1) [Eq. (47)].
ligand couples the two tungsten atoms via
The ",I4short W-N bonds (average length 164 pm) and two relatively long N-N bonds (149 ~ r n ) [ ' (Fig.
~ ~ ] 23).
Fig. 24. Structure of the ion pair [K([18]crown-6)][F5W(NC1)].Crossed spheres
are 0 atoms, smaller white spheres in the crown ether C atoms. The white
spheres which are bound directly to the W atom are F atoms.
Fig. 23. Structure of the /c-1sotetrazenido(4-) complex [CI5W(/~-N.,)WCI,JZ-.
The white spheres are CI atoms.
The long N-N bonds signal the ready thermal cleavability
of nitrogen. The [N4I4- bridge ligand can derive from the
deprotonated, unknown isotetrazene, which is isoelectronic
with urea and with the well-characterized tetrazene,
2 ReNC1,
Starting materials for ligand exchange reactions which
proceed with retention of the nitrido function are mostly
chloronitrido complexes in which the C1 ligands can, by suitable choice of reaction partners, be partially or completely
substituted. We shall now describe some typical examples
from the wide spectrum of preparative possibilities.
A suitable reagent for the preparation of fluoronitrido
complexes is anhydrous silver fluoride in acetonitrile. Treatment of the chloronitrido complex with this reagent leads to
the formation of extremely moisture-sensitive fluoronitrido
silver complexes in the form of their readily soluble acetonitrile solvatesr'061[Eq. (48)].
+ 4AgF
2 [Ag(CH,CN),][MoNF,]
AngeM. Chem. I n t . Ed. Engl. 1992, 31, 955-978
+ 3AgCI
+ 8 F,
[ReNF, . ReF,(NCI)]
+ 7 CIF
The two Re components are coupled by an asymmetric
fluorine bridge with Re-F distances of 188 and 228 pm, res p e c t i ~ e l y [(Fig.
' ~ ~ ~25).
3.3. Substitution Reactions
A crystallographically characterized ion triplet is shown in
Figure 11. Occasionally chloride nitrides have also been allowed to react with elemental fluorine, whereby the nitrido
function remains intact despite the drastic reaction conditions. Thus, reaction of ReNCI, with fluorine yields mainly
red [ReNF, . ReF,(NC1)]['091 [Eq. (SO)] besides small
amounts of yellow crystals of ReNF,, which is also accessible from ReF, and trimethylsilyl azide.['081
Fig. 25. Structure of the molecular complex [ReNF, . Re(NCI)F,]. With exception of the labeled C12 atom. white spheres are F atoms.
The long Re1 -F bond is in the trans-position to the nitrido ligand [ r N : I 3 - , which therefore exercises a much larger
trans influence than the nitreno ligand [=N-C1I2-. The short
Re2- F bond remains almost unaltered compared to the free
molecule [ReF,(NCI)J.['081
A CI/Br ligand exchange can generally be achieved quite
readily with the help of trimethylsilyl bromide, especially if
the chloro ligands are complexochemically enfeebled by excess negative charges [Eq. (51), M = Mo, Wl.[llol
+ 4BrSiMe,
+ 4CISiMe,
+ 4EtBr
80 C
Occasionally ethyl bromide is also suitable for ligand exchange; however, longer reaction times of up to eight days
are required["'] [Eq. (52)].
By comparison, the reactions of thiophenolates with nitridorhenium(v) complexes present no problems. Thus, complexes of the type [ReNCI,(PR,),] (PR, = PPh,, n = 2 or
PMe,Ph, n = 3) react with 1,3,5-triisopropylthiophenol
(Htipt) in the presence of triethylamine smoothly according
to Equation (57),['
and with sodium thiophenolate according to Equation (58) to give the thiophenolato complexes,[' 151
+ 4EtCI
Nitrido derivatives of molybdenum and tungsten with
alkoxy ligands can be prepared by reaction of the tert-butyloxy derivatives obtainable from the metathetic reaction
(37) with alcohols bearing sterically less demanding
771 [Eq. (531.
residues R[763
+ 2Htipt 4
[ReN(tipt),(PR,),] + 2HCI
+ 4Na(tipt) +
Na,[ReN(tipt),]. 2THF + 2NaCl
+ nPR,
In some cases substitution reactions can also be achieved
by exploiting the chelate effect of neutral multidentate donor
molecules. Thus, AsPh,[ReNCI,] reacts with 2,T-dipyridyl
in dichloromethane solution via exchange of a chloride to
give the uncharged molecular complex [ReNCl,(bpy)][' 161
[Eq. (59)], while reaction of [OsNCI,J- with the tridentate
chelate terpyridyl in boiling acetone even leads to the cationic nitrido complex [OsN(terpy)CI,J [' ' 71 [Eq. (60)].
[(~BUO),MEN:]+ 3ROH
+ 3tBuOH
The complex [NBu,][OsN(OSiMe,),],['
readily accessible by reaction of [NBu,][OsNCI,] with NaOSiMe, , has
proven to be an excellent starting material for the synthesis
of organo-substituted nitridoosmium(v1) complexes. Alkylmagnesium and -aluminum compounds with R = CH,,
CH,Ph, CH,CMe,, and CH,SiMe, are suitable as alkylating reagents.["'] The crystal structure analysis of
[NBu,][OsN(CH,SiMe,),] shows the anion in a slightly distorted C, symmetry with the nitrido ligand in the apical
position ( O s s N =163 pm) and the methylene C atoms in
the basal plane (0s-C = 212-214 pm).[1131
Attempts to prepare anionic nitrido complexes
[(tBuO),M=N:] - starting from the chloronitridometalates
(M = Mo, W) were unsuccessful owing to the bulkiness of
the organic residues. Instead, these reactions proceed according to Equation (54).[' In contrast, no problems were
encountered in the synthesis of the corresponding anionic
phenolato complexes["41 [Eq. (55)].
+ 3NaOtBu
+ 4NaOPh
+ PPh,C1 + 3NaC1
+ 4NaCl
+ 3HO-CH2-CH,-OH
+ 3DBU . HCI + NH,
DBU = 1,8-diazabicyclo[5.4.O]undec-7-ene
+ CI-
+ 2C1-
Other than in the terpyridyl complex [MoNCI(N,),( t e r ~ y ) ] , [in
~ ~which
the nitrido ligand is in the cis-position
to the terpyridyl chelate ligand (Fig. lo), in the osmium complex the nitrido ligand is in the frans-position to the N atom
of the central terpyridyl
(Fig. 26). This leads to an
Fig. 26. Structure of the cation [OsNCl,(terpy)]+. With exception of the labeled CI atoms, white spheres are C atoms.
Attempts to convert the chloride nitrides into alkoxidonitrido complexes with alcohols led, even in the presence of
bases, via loss of the nitrido group to formation of homoleptic alkoxido complexes. If diols are used as reactants, trischelates of molybdenum(v1) can be
[Eq. (56)].
+ bpy
+ terpy
enormous stretching of this 0s-N bond by 85 pm compared
to the two other 0s-N distances to the terpyridyl ligand.t1'71
As in other d2-configurated complexes with n-bound ligands["*] the nitridoosmium complex is also diamagnetic,
corresponding to a marked energetic separation of the dorbitals according to d,, < d,,, dyz.[1171
Also the p-nitridoruthenium complex K,[Ru,NCI,(H,O),] can be converted in a slow reaction, via replacement
of the chloro ligands by ethylenediamine, into the cationic
complex [Ru,N(en),]CI, . H,0['191 (Fig. 27). The bridging
of the two ruthenium atoms by one of the ethylenediamine
chelates leads to only a slight bending of the Ru-N-Ru bond
angle to 174.6".
Angew. Chem. Inl. Ed. Engl. 1992, 31, 955-978
Fig. 27. Structure of the cationic p-nitrido complex [ R ~ ~ N ( e n ) ~ 1White
spheres are C atoms.
Substitution of the chlorine atoms of the polymeric chloride nitrides MoNCI, and WNCI, by sterically demanding
diphenylamido groups led to the first monomeric molecular
nitrido complexes with four-coordination at the metal
atom[12n1[Eq. (61)]. The steric requirements of the NPh,
+ 3LiNPh,
+ 3LiCI
treated with excess sodium cyanide, thus affording the pentacyanonitrido complex [OsN(CN),]' - .1121J
The structure of the cyano complex corresponds entirely
to that of the long-known [OSNCI,]~-ion;11zz1the trans
influence of the nitrido ligand is reflected in the long 0s-C
bond to the trans C N - ligand, which, with 235 pm, is 27 pm
longer than the 0s-C distances of the equatorially arranged
cyano ligands.['211In solution at room temperature the complex shows an intense phosphorescence at 550 nm upon excitation with light of wavelength 300-400 nm.11211
The remarkable inertness of the nitrido function in technetium complexes towards proton-active reagents permits a
wide variety of substitution reactions. Starting materials for
this purpose are frequently the readily accessible nitrido
complexes AsPh,[TcNCI,] with C,, symmetry of the [TcNC14]- anion[1231
or [TcNCI,(PPh,),] with trigonal-bipyramidal geometry at the technetium atom and trans arrangement
of the phosphane ligands. The structure of this molecular
complex was first elucidated only recently[124J(Fig. 29).
groups lead to a structural peculiarity: whereas in all other
nitrido complexes the large n-electron density of the M E N :
bond leads to :N=M-X bond angles which are larger than
those of the regular geometric arrangement, the N z M o -N
bond angles in the structure of [MoN(NPh,),] (100.5") are
Fig. 29. Structure of the nitrido complex [TcNCI,(PPh,),]. The C atoms are
scaled down.
Thus, for example, cyclam (Scheme 8) reacts with [TcNCI,(PPh,),] smoothly according to Equation (62) with dis-
Fig. 28. Structure of the diphenylamidonitrido complex [MoN(NPh,),]. The
white spheres are C atoms. One N atom is masked by a phenyl ring.
smaller than the ideal tetrahedral angle, while the bond
angles N-Mo-N (1 14.9- 119.6") are correspondingly larger
than the tetrahedral angle['2n1(Fig. 28). The Mo-N bond
length (163.4 pm), however, remains unaltered.
In the case of nitrido complexes whose nitrido function is
not sensitive towards proton-active reagents such as water,
alcohols etc., there are far more numerous possibilities for
derivatization by substitution of the halogeno ligands. This
is true in particular for the nitrido complexes of technetium
and osmium.
Thus, e.g., reaction of AsPh,[OsNCI,] with sodium
cyanide in THF/methanol furnishes the mixed complex [OsN(CN),(OH)]2- in which the OH- ligand is trans to the
nitrido 1igand.l'
Owing to the strong trans effect of the 0s-N group, the
OH- ligand is readily replaced by CN- when the complex is
Angen'. Cliem. Inf. Ed. Engl. 1992, 31, 955-978
Scheme 8. The cyclam ligand.
placement of the two phosphane ligands and one chloro
ligand and formation of a cationic complex.['25]
+ cyclam
+ 2PPh,
With the cyclam derivative 1,4,8,1l-tetraazacyclotetradecane-5,7-dione (H,L), substitution with elimination of
HCI already takes place at 40 "C in CH,CI,/EtOH [Eq. (63)].
+ H,L + 3H,O
+ 2HC1+
In [TcN(L)(H,O)] .2H,O the technetium is surrounded in
a distorted octahedral fashion by the four equatorially arranged N atoms of the cyclam hgdnd (distances Tc-NH
213 pm, Tc-N 205 pm) as well as axially by the nitrido ligands (TEN 161 pm) and the 0 atom of the water molecule.
Owing to the strong trans influence of the nitrido ligand the
aqua ligdnd is only very loosely bound.['25J
Also other proton-active chelating reagents such as
dithiacarbamide or azomethine derivatives react with
[TcNCI,(PPh,),] in a very similar way with elimination of
HCI. Thus, the reactions with the carbamide derivatives
H,L3 and HL2 in boiling benzene lead to the molecular
Scheme 9. Structural formula of the tridentate ligand H,ecbap (N(2-ethoxycdrbonyl-3-oxobut-1 -en( l)yl)-2-aminophenol, non-IUPAC name).
S Me
S Me
Fig. 31. Structure of the nitrido complex [TcN(ecbap)(PPh,)]. The white
spheres are C atoms, the crossed spheres 0 atoms.
[Eq. (65)],
+ H,L3
+ 2HL2
[Eq. (64)]
+ PPh, + 2HCl
+ ZPPh, + 2HCl
In these two complexes the technetium is surrounded in a
distorted tetragonal-pyramidal fashion, with the nitrido ligand in the apical position['261 (Fig. 30).
in aqueous medium, since, in each case, the nitrido ligand is
insensitive to hydrolysis. This could be exploited in the remarkable synthesis [Eq. (67)] of the nitridooxotechnetium
complex coupled via 0x0- and oxalato bridges['281(Fig. 32).
+ 6H,C,O, + 2H,O
(A~P~,).,[TC,N,O,(C,O~)~] 16 HCI
Fig. 30. Structure of the dithiocarbamido complex [TcN(Me,CNNC(S)SMe),]. The white spheres are C atoms, the crossed spheres S atoms.
Analogous results could be achieved with the tridentate
chelating reagent H,ecbap (Scheme 9) in
[Eq. (6611.
[TCNCI,(PP~,)~] H,ecbap
+ PPh, + 2HCI
The tridentate ligand coordinates with the ligator atoms
O N 0 as rigid planar
(Fig. 31).
The chloronitrido complexes of technetium, [TcNCIJ
and [TcNCI,]*-, are suitable for substitution reactions even
Fig. 32. Structure of the tetranuclear nitrido complex [Tc,N,O,(C,O,),]'~.
The white spheres are C atoms, the crossed spheres 0 atoms.
With hydrogen peroxide it was even possible to synthesize
the first nitridoperoxo complex['291[Eq. (68)J.
2 Cs,[TcNCI,]
+ 5 H,O,
+ 6HC1 + 2CsCI + 2H,O
Aflgew. Chem. Int. Ed. Ennl. 1992, 31, 955-978
In this reaction the substitution is accompanied by an
oxidation of the technetium from Tc"' to Tc"". The structure
of the anion (Fig. 33) can be best described in terms of a
Fig. 33. Structure of the nitridoperoxo complex [TcNCI(O,),]-. The crossed
spheres are 0 atoms.
distorted pentagonal pyramid,['"] but to a first approximation also in terms of a pseudotetrahedral arrangement.
In contrast, the substitution reactions of the [TcNCl,]ion are more frequently accompanied by a reduction of
the technetium to Tcv. Examples are the reaction with 8quinolinethiol, which leads to the molecular complex
[TcN(C,H,NS),] with fivefold coordinated technetium
atom[130]or the reaction with 1 ,l-dicyanoethene-2,2-diselenolate [(NC),C = CSe,12-, in which the technetium(v)
complex [TcN(Se,C = C(CN),),]2- with tetragonal-pyramidal coordination is formed. The chelate ligands bound via
their selenium atoms are arranged equatorially, whereas the
nitrido ligand occupies an apical position."
gen to the corresponding rhenium(v1) complex [ReN(tipt),] - .I1 5 1
The reduction of MoNC1, in T H F solution also proceeds
with retention of the tetrameric structural unit of the solvates
of MoNCl,, as has been demonstrated crystallographically,
e.g., with the dibutyl ether solvate [MoNCI, . O BU , ],.[' ~ ~ ~
Upon reaction of [MoNCl, . thfl, with an equivalent
amount of sodium naphthalide in the presence of
[I 51crown-5 the tetrameric molybdenum(v) complex
[{MoNCl, . thf},14- could be isolatedf1371
[Eq. (70)].
+ 4NaC,,H, + 2([15]crown-5) + 4 T H F
[Na([15]crown-S)Na(thf)],[{MoNCl3~thf}, . 2 T H F + 4Ci,H8
[{MoNCI, . thf},]
The compound is an ionic ensemble which is formed by
direct contacts of chlorine atoms of the complex anion
[(MoNCl, . thf},14- each with two differently associated
(Fig. 34).
3.4. Redox Reactions
Investigations of redox reactions of nitrido complexes,
particularly of molybdenum, have attracted special attention
in model concepts concerning natural N,-assimilation. The
molybdenum atom participates in redox processes from the
N,-fixation at the metal center through to the nitrido complex Mo-N: .I'3 2 , 1 3 3 1 Electron transfer reactions of nitridoosmium complexes have recently also gained importance.
The nitridomolybdenum(vi) complex [MoNCl,]- can be
readily converted into the corresponding molybdenum(v)
complex in CH,Cl, solution even with iodide as reducing
agent['341[Eq. (69)]. According to the crystal structure anal-
+ 3[PPh,Me]I
2 [PPh,Me],[MoNCl,]
+ [PPh,Me]I,
ysis the [MONCI,]~-ion retains the C,, symmetric structure
of the [MoNCI,]- ion. The Mo-N bond length remains virtually unchanged (1 63.4 pm), whereas the Mo-C1 distances,
owing to the higher negative charge of the ion, are 5.5 pm
longer than in the singly charged ion. This bonding situation
is also reflected in the results of the EPR spectrum, in which
the g , parameter with a value of 1.984(1) is significantly
lower than in the corresponding oxomolybdenum(v) compounds, and can be interpreted as an expression of the strong
M o r N x bond.['341
On the other hand, the sterically very crowded thioarene
complex [ReN(tipt),]' - (tipt = 2,4,6-triisopropylbenzenethiolate) can be oxidized very easily with atmospheric oxyAngeu. Clicm. Int. Ed. Engl. 1992. 31, 955-978
Fig. 34. Structure of the ionic ensemble [{Na([l5]crown-5)Na(thf)),l[{MoNCI, . thf],]. The large white spheres are CI atoms, the small white
spheres C atoms, the crossed spheres 0 atoms.
The sodium ions Nal are, on the one hand, each coordinated by five 0 atoms of the crown ether molecule, and, on
the other, by two chlorine atoms of the tetrameric anion, so
a coordination number seven results for these sodium ions.
The sodium ions Na2 are square-pyramidally coordinated,
namely by four chlorine atoms, which form the pyrimid base,
while the apical position is occupied by the 0 atom of a
tetrahydrofuran molecule.
The Mo-N distances in the [{MoNCI, . thf}J4- ion can,
as in the comparable molecular complexes, be interpreted as
triple and single bonds. However, from the EPR spectrum, in
comparison to the spectrum of the (PPh,Me),[MoNCl,]
complex with monomeric anion, a doubling of the orbital
moment can be concluded. This is a result of the weakening
of the Mo=N bond, which is determined by the bridging
function of the nitrido ligand.f'371
201 is formed under reducing
conditions in the reaction of WNC1, with LiNPh, [cf.
Eq. (61)] in the presence of butyllithium in T H E According
97 1
The cationic nitridoosmium(v1) complex [OsN(terpy)Cl,] can also be very readily reduced electrochemically
through four oxidation states to give the amminoosmium(r1)
complex[' 'I [Eq. (72)], which is easily oxidized by atmospheric oxygen to the cationic complex [Os"'(NH,)(terpy)CI2]+.['"1
The known phosphoraniminato complexes of various
transition metals which were discussed in detail in a recent
review['401contain the structural group MNPR, , mainly in
a linear arrangement (J); but M-N-P angles of up to 130"
are also known (K).
Scheme 10. Bond situation in [W,N,(NPh,),(OBu),]
Phosphoraniminato complexes are compounds containing the ligand [NPR,]- H, which is isoelectronic with compounds of the type OPR, I, and thus far could not been
observed as uncoordinated ion.
to the crystal structure analysis['201 (Fig. 35) the two tungsten atoms W1 and W2 are coupled one under the other via
a W-W single bond (253.5 pm) (corresponding to d' configuration). The two remaining tungsten atoms have the oxidation number + V I ; they are coupled via two unusual nitrido
bridges (NI and N2) with the W-W dumbbell. The nitrido
ligdnd N2 has a W-N-W bond angle of only 91.3", the
smallest such angle observed so far. The nitrido ligand N1
has a T-shaped bridging function, which has already been
observed occasionally. The bonding situations are best formulated as in Scheme 10.
3.5. Nitrides as Educts for the Synthesis
of Phosphoraniminato Complexes
Fig. 35. Arrangement of the framework atoms of the complex
without phenyl groups of the amido ligands. White
spheres are C atoms, crossed spheres 0 atoms.
+ 4e + 3 H'
N =PR,
- M
0 'PR,
Other complexes with the T-shaped bridging ligand are
shown in Scheme 11.
In all these cases we have complexes in which the 11,-N
ligdnd couples metal atoms, which, in turn, form metalmetal bonds. An exception is the pentameric complex
[ ( ~ B u C H , ) , T ~ N ] , [(cf.
~ * ~Section 2.3).
Reactions of nitrido complexes with phosphanes are often
suitable for their syntheses; these are interpreted as nucleophilic attack of the phosphorus atom at the nitrido lig(Scheme 12).
Scheme 12. Nucleophilic attack of the P atom at the nitrido ligand
Scheme 11. Complexes with nitrido ligands which form T-shaped bridges.
The previously mentioned complex [AsPh,],[OsN(CN),(OH)]['211 (cf. Section 3.3) has proven to be a strong
oxidizing agent in flash photolysis experiments with laser
excitation of 355 nm. The self-quenching process of the excited state can be formulated as in Equation (71).r'2i1
[ O S ~ ' ~ N+] *[N-OsV1]
[ O S ~ N . . . N ~ O S ] 2[Osv=N]
The phosphorus atom is oxidized (change in oxidation
state by two), the metal correspondingly reduced.
More recent examples for the phosphoraniminato complexes accessible in this way are the reactions of ReNCI, with
triphenylphosphane to give [ (Ph,P)Cl,Re(NPPh,)],['
formation of PPh,[MoCI,(NPCI,)] from PPh,[MoNCI,]
and PC1,/PCI,,[L4z1 and the reaction of MoNCI, with
trimethylphosphane in CH,CI,, which already takes place at
[Eq. (73)].
+ 4PMe,
Anger.. Chem. Inc. Ed. Engl. 1992, 31, 955-978
The Mo-N-P bond angle in the structure of the first
cationic phosphoraninimato complex (167.6")['431 corresponds to that in the most frequently observed structure type
On prolonged standing in CH,CI, solution, this Mo'"
complex undergoes a remarkable transformation; besides
the bis(trimethy1phosphane)iminium ion, a p-nitridomolybdenum(1v) complex is formed['431[Eq. (74)]. In the structure
[Me,PNPMe,]+[{ MoCl,(PMe,),},N]-
+ 2PMe,
of the anion (Fig. 36) the Mo-N-Mo axis is almost symmetrical and nearly linear (bond angle 175.3").['431
A further example of a cationic phosphoraniminato complex could be prepared by reaction of the terpyridyl complex trans-[OsV'(terpy)C1,N]C1 with triphenylphosphane
[Eq. (77)] and was crystallographically characterized as the
PF; salt.['"81
+ PPh,
In this case the 0s-N-P bond angle of 132.5" lies at the
lower limit of the previously known values. Only in the complex [Mo(NO)(NPMePh,)(dttd)] [dttd = 2,3,8,9-dibenzo1,4,7,1O-tetrathiadecane(2-)] is it somewhat smaller
(129.7°).[1491Also remarkable is the length of the 0s-N bond
which corresponds to a single bond.
of 209
3.6. Nitrides as Educts for the Synthesis of M-N-S
Some nitrido complexes can be converted by reaction with
elemental sulfur or with S,Cl, into thionitrosyl complexes
[Eq. (78), (79)]. Suitable educts are dithiocarbamato complexes of molybdenum [MoN(S,CNR,),] with sevenfold coordinated molybdenum atom as well as nitrido complexes of
rhenium and osmium.[1501
Fig. 36. Structure O f the anionic Ic-nitrido Complex [{MoCI,(PMe,),},N]~.
The uhite spheres are C1 atoms, the black spheres P atoms, the spheres with
central dot C atoms.
Cyclic metallaphosphazenes could be prepared in good
yields for the first time by reaction of bis(aminodiorganophosphane)iminium chloride with the chloride nitrides MoNCI, or WNC1,[144-1451 [Eq. (75)]. Crystal struc-
R, p H N \ p
ture investigations of the acetonitrile solvates revealed the
presence of almost planar MN,P, rings with delocalized P-N
bonds.[145. Still unclear is whether the nitrogen of the
ammonium chloride that is formed stems from the metal
chloride nitride or from an amino function of the iminium
ion. The synthesis of the cyclotungstaphosphazene
Cl,WNPPh,NPPh,N starting from tungsten hexachloride
and [H,NPPh,NPPh,NH,]Cl, which leads to elimination of
HC1, would suggest that it originates from the MNCI, component .['461
The formation of bis(ph0sphane)iminium ions from phosphoraniminato complexes was previously only observed on
starting from a molybdenum complex with cis phosphoraniminato ligand~"~'][Eq. (76)].
+ +S,CI,
+ PR,
According to structural investigations the thionitrosyl
ligand is bound in linear arrangement M = N = S with the
metal center (for a review see Ref. [151]).
The reaction of ReNCl, with tetrasulfur tetranitride leads
to an interesting product. Initially, a chloro-bridged polymerized product [ReCI,(NS)(NSCl)], is formed which can
be resolved with tetraphenylarsonium chloride in
dichloromethane to give the dianion [ReCI,(NS)(NSCI)]2-.11521
According to the structural analysis (Fig. 37)
(M = Mo, W)
Cheni. In1 Ed. EngI. 1992. 31. 955-978
Fig. 37. Structure of the anionic chlorothionitrenethionitrosyl complex
[CI,Re(NSCl)(NS)]2-. The white spheres are CI atoms.
the anion contains both a thionitrosyl as well as a chlorothionitrene ligandr1521
(cf. also Section 2.7).
Also remarkable is the reaction of molybdenum chloride
nitride with trithiazyl chloride, which is in equilibrium with
its monomer in solution. A cyclothiazenomolybdenum
derivative [Eq. (SO)] is formed whose chloro bridges are detached with tetraphenylphosphonium chloride to give the
monomeric anionic derivative[' 531 [Eq. (Sl)].
+ 4:N=S-C1
+ 2PPh4C1
+ 2C1,
The largest deshielding is observed in the [MoNCIJ ion
with fivefold coordinated molybdenum, the least deshielding
in [MoN(S,CNMe,),] with coordination number seven.
"N-NMR investigations have been carried out on a few
nitrido complexes of molybdenum, tungsten, and rhenium[',
(Table 2). Also these measurements show a
Table 2. "N-NMR data of nitrido complexes.
According to the crystal structure analyses of the AsPhf
salt['541and the PPh; salt,['551which are not isotypic, the
Fig. 38. Structure of the anion [MoCI,(N,S,)]-. The white spheres are CI
molybdenum atom is component of a planar six-membered
MoN,S, ring (Fig. 38), whose S-N distances are almost
equal in length and can be described in terms of delocalized
For a review of ring compounds of this type see Ref. [I 561.
Scheme 13. Resonance structures of the planar six-membered MoN,S, ring.
6 [a1
[ReCI,(' 'N)(PnPrPh,),]
lMo(' 'N)(S,CNEt,),l
truns-[MoCl(' 'N)(dppe),]
truns-[MoBr(' 'N)(dppe),]
[W,('SN)CI,. CH,CN]
[(W("N)CI,. CH,CN),]
PPhJWL' 5N)Cl,ol
21 1.2
[f 591
[a] Chemical shifts relative to nitromethane. [h] dppe = Ph,PCH,CH,PPh,.
strong dependence of the values of the chemical shift on the
coordination number at the metal center and on the type of
ligands. The strongest shielding is experienced by the nitrido
ligand in the complex [MO('~N)(S,CNE~,),]with sevenfold
coordinated molybdenum atom, which has its complete
equivalent in the large extent of the shielding of the "Mo
nucleus in this compound (see above). The N-nuclei of the
nitrido ligands in the tungsten compound are the most
strongly deshielded. Tungsten is thus a particularly strong
n-electron acceptor towards the nitrido ligand, as is also
reflected in the IR spectra by very short wave W-N stretching frequencies and correspondingly large valence force constants. Thus, e.g., in the ions [MNClJ an M-N valence
force constant of 8.00 Ncm-' is calculated for M = Mo and
of 8.15 Ncm-' for M = W.[71
5. Catalytic Activity of Nitrido Complexes
4. NMR Spectroscopic Investigations
Very little has been reported in the past few years concerning NMR spectra of nitrido complexes. 95Mo-NMRspectra
disclose a wide range of the values of the chemical shift
through its dependence on the coordination number, on the
electronegativity of the ligands, and on the charge of the
complexes['571(Table 1).
Nitridomolybdenum(v1) complexes such as [MoNCI,(OPPh,),] or [NBu,][MoNCI,] together with ethylaluminum
chloride as cocatalyst are very efficient catalysts for olefin
For example, pent-2-ene is rapidly converted
into but-2-ene and hex-3-ene [Eq. (82)], whereby the high
Fdble 1. Chemical shifts b and line widths h of nitrido complexes [157] in 95MoNMR spectra [a].
MoNCI, [bl
- 121
- 103
h [Hzl
< 20
[a] Measurements in CH,CI,, external standard 2 M NaJMoO,] in D,O,
pH 11. [b] Solution in acetonitrile. The complex [{MoNCl, . CH,CN},] is
formed in this solvent.
+ C,H,-CH=CH-C,H,
(27 %)
(46 Y o )
(27 Yo)
initial rate of the reaction is impressive. It is assumed that the
course of the olefin metathesis proceeds via the initial formation of a carbene complex. An organylation of the MoNCI,
precatalyst followed by a-H-elimination with reduction to an
active carbenenitridomolybdenum(1v) complex fragment is
conceivable as catalyst formation reaction['"] [Eq. (83)].
+ (2 + x)C,H,AICI,
+ 2[CI,Al + OPPh,] + C,H,
Angew. Chem. Int. Ed. Engl. 1992, 31, 955-978
This assumption is supported by the isolation of the carbeneoxotungsten complex [{WO(CHCMe3)(PMe3)Clz}2],
which likewise catalyzes the olefin metathesis, albeit less efficientIy.[1611
On the basis of theoretical considerations R. Taube and K.
Seyferth came to the conclusion that the strong x-donor
action of the nitrido ligand favors the form M of the cdrbene
complex with d:y configuration over the form L, which is
essential for the subsequent reaction with the olefin.[1621
can then lead via the intermediate step of a metallacycloalkane to the metathesized olefin.['6z1
The nitrido complexes, which have hitherto only been little exploited for catalytic purposes obviously present a challenge for the future. Whereas the development of homogeneous catalysts based on 0x0 complexes, for which
[CH,ReO,] is mentioned as key substance, has already led to
excellent results in the metathesis of functionalized
ole fin^,['^^] in olefin epoxidation with H202[165]
or even in
the olefin synthesis from aldehydes, cyclic ketones etc.,"661
the catalytic potential of nitrido complexes still awaits recognition. Optimism accrues from a comparison of 0x0 function
and imido function, which are isolobal (Scheme 14).
0 to'M - 0 :
Scheme 14. Comparison of the isolobal 0x0 and imido ligands.
Carbenimido complexes such as [Mo(NtBu)(CHtBu)(OR2)][1671
(R = CH(CF,),), like 0x0 complexes, have also
proven to be effective as catalysts in olefin metathesis. Recently, the catalytic activity in the metathesis of terminal and
functionalized olefins of a carbyneruthenium complex,
[Ru(CtBu)(CHtBu)(ORz)1["81 (R = C(Me)(CF,),) was reported on for the first time. Since the carbyne function is
isolobal with the nitrido ligand (Scheme 15), corresponding-
Scheme 15. Comparison of the isolobal carbyne and nitrido ligands.
ly modified properties are also to be expected for nitrido
complexes. The above mentioned examples are a good indication of this being the case.
Angiw. Chem I n t . Ed. Engl. 1992, 31, 955-918
6. Bonding Situations in Multinuclear Nitrido
As has already been shown, binuclear, oligomeric chain or
ring-shaped as well as polymeric complexes can be formed by
the bridging function of the nitrido ligand. The nitrido
bridge can be symmetrical or asymmetrical, linear or bent
(see Scheme 1).
R. Hoffmann et al. have devoted several studies to the
bonding situations in multinuclear complexes with nitrido
bridges[". 169, I7O1 and have examined the geometric and
electronic properties using extended Hiickel MO calculations. They also compare the nitrido bridges with fluoro,
0x0-, and carbido-bridges.
In the case of binuclear porphyrin-analogous model complexes [(N,)Fe-X-Fe(N,)] with X = 0, N, C, the calculat i o n ~ [ ' show
~ ~ ] that when X = 0 two and when X = N one
d electron of the Fe ions is located in antibonding Fe-X-Fe
orbitals. Due to a bending of the bridge the orbitals of these
electrons are energetically lowered, but at the same time an
occupied non-bonding level is slightly raised. This indicates
that a bent 0x0 bridge leads to an energetically more favorable situation, whereas in the case of the nitrido bridge the
lowering of the only singly occupied antibonding orbital cannot compensate the opposing effect in the non-bonding MO,
so that the frequently observed linear arrangement is preferred. However, the calculated bending of Fe-0-Fe bridge
is in fact only observed when this is stereochemically allowed. Thus, the 0x0 bridge is linear for example in the
phthalocyaninato complex [ (pc)Fe-0-Fe(pc)].
In the carbido complex [(N,)Fe=C=Fe(N,)] the antibonding Fe-C-Fe MOs are unoccupied, so a linear bridge
results, as is found in [(tpp)Fe=C=Fe(tpp)][1711and
[(pc(Fe=C=Fe(pc)].[' 721 Naturally, the Fe-X bond strength
increases with decreasing number of antibonding electrons.
Hence, the strongest Fe-X bond is found in the carbido
In binuclear complexes the symmetrical M = N = M bridge
with equal M-N distances is preferred, whereas in polymeric
and most of the oligomeric nitrido complexes the nitrido
bridges M=N-M are, as a rule, asymmetrical. In addition,
a pyramidalization of the equatorial plane L,M ( n = 3, 4)
formed by the central metal atom and the cis oriented ligdnds
R. Hoffmann et al.[1701show, in the case of complexes
with a do- or d'-electron configuration at the central atom,
that this is the result of a second-order Jahn-Teller effect, in
which, due to a HOMO-LUMO interaction, the HOMO is
lowered with respect to its energy and the LUMO is raised.
The HOMO-LUMO interaction is thereby all the more
stronger the smaller the energy difference between the two
MOs. This increases with increasing electronegativity of the
bridge ligand X, so that also the asymmetry of the bridge
decreases along the series X = C, N, 0, F. This is confirmed
by structure analyses. Fluoro bridges are usually symmetric,
nitrido bridges, on the other hand, asymmetric. Polynuclear
complexes with carbido bridges have so far not been synthesized. Here, highly asymmetric bridges should occur.
Since the highest occupied MOs are two degenerate M-X
x-orbitals, the gain in energy by the second-order JahnTeller effect leads in the first instance to a strengthening of
the n-bond, whereas the M-X o-bond is influenced only
that the tetragonal-bipyramidal structure with the nitrogen
slightly. Rather it stabilizes the symmetrical bridge bonding.
atom in the apical position is only about 12-13 kJmol-'
In the case of the hitherto unknown polymeric nitrido
more stable than the trigonal-bipyramidal alternative. This
complexes with two or more free d-electrons on the transiexplains why, for example, steric effects such as are effected
tion metal, according to calculations symmetrical nitrido
by the phosphane ligands in [TcNCI,(PP~,),],"~~~
can stabibridges should occur, since here the antibonding M-N n*
lize the trigonal-bipyramidal arrangement.
levels are filled and the Jahn-Teller effect cannot be effective.
For a trigonal bipyramid with the N atom in the axial
Similarly as in the case of the polymeric nitrido complexes
position there is likewise a local minimum, which however
[MNL,], and [MNL,],l, a second-order Jahn-Teller effect
lies at higher energy than in the case of the equatorial aralso leads in the case of tetrameric complexes to stabilization
of the less symmetric structure with alternating M-N disA pseudorotation should, according to calculations, be
tances. Calculations show, in the case of the model comeasily possible in the case of a do-d2 electron configuration
pound [{MoNCI,(OH,)},] an energy gain of about
of the metal; in the case of d3-d7 electron configuration it is
193 kJmol-' on going from the complex with symmetrical
symmetry forbidden.
nitrido bridges to the complex with alternating Mo-N distances, in which the weakly bound aqua ligands are then
arranged trans to the Mo-N triple bond.
7. Outlook
In the transition from a nitrido bridge to an 0x0- or fluoro
bridge the extent of distortion decreases with increasing elecNitrogen is an extremely versatile element, which, owing
tronegativity of the bridge ligand, since the energy difference
to its manifold bonding and coordination possibilities, also
between HOMO and LUMO increases and therefore the
promises a diversified chemistry in the future. Thus, the
second-order Jahn-Teller effect becomes weaker. (NbF,],
chemistry of the nitrido complexes has developed extensively
in the past few years. This is due in the first instance to the
has indeed symmetrical fluoro bridges. Likewise, in the cyclic
tetrameric phosphazenes [PNX,], the corresponding ormany new synthetic methods which have opened up entries
bitals are energetically so different that no second-order
to nitrogen compounds. It is expected that this development
will continue in the coming years, since the known synthetic
Jahn-Teller effect occurs, thus explaining the equalized P-N
methods are still not exhausted and surely new ones will be
bond distances.'221
Interesting is a comparison of the polynuclear nitrido
added. Possibly also new nitrido complexes of the previously
little investigated main group elements will be found. Also in
complexes and linear and cyclic (CH,), polyenes.[221It
emerges that the known nitrido complexes are related to the
the area of catalysis it can be expected that nitrido complexes
polyene analogues polyacetylene, allyl and pentadienyl
will find application as effective catalysts. Finally, the search
cation and cyclooctatetraene. This structural relationship is
for preceramic materials will give new impetus to the chemexplained by the similarity of the n-systems, which is evident
istry of the nitrides of main and sub-group elements.
in particular in nitrido complexes, since the difference in
We thank the Deutsche Forschungsgemeinschaft and the
electronegativity between metal and bridging N atom is
Verband der Chemischen Industrie for generous support of our
work. Thanks are also due to our co-workersfor their enthusiThus, it appears that binuclear nitrido complexes with
astic and invaluable cooporation. We also thank Mrs. E. Nisymmetrical nitrido bridges correspond to the allyl system.
quet for preparing the SCHAKAL diagrams.
Trinuclear complexes such as ~,N,C11,]2- ["I containing
the structural group W=N-W-N=W are comparable with
Received: December 30, 1991 [A 869 IE]
the pentadienyl cation, [{MONCI,),][~~~
corresponds to cyGerman version: Angew,. Chem. 1992, 104, 978
clooctatetraene, and the polymeric nitrido complexes [ReNCl,]n,[l 7 3 1 [MONCI;],[*~~
and [(RO),WN],r771have alternating M-N distances, as is the case in polyacetylene.
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metali, transitional, complexes, nitride
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