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Phosphorus Ylides in the Coordination Sphere of Transition Metals An Inventory.

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I921 H. Brunner, D. K. Rastogi, BUN. SOC.Chim. Belg. 89 (1980) 883.
[93] I. Bemal, W. H. Ries, H. Brunner, D. K. Rastogi, Inorg. Chem., in
press.
[94] D. K. Rastogi, S. Rastogi, Polyhedron I (1982) 233.
[95] H. Brunner, G. Riepl, R Benn, A. Rufinska, J. Organomet. Chem. 253
(1983) 93.
I961 T. Hayashi, K. Yamamoto, M. Kumada, Tetrnhedron Left. 1974, 4405.
[971 N . C. Payne, D. W. Stephan, Inorg. Chem. 21 (1982) 182.
1981 T. Hayashi, K. Yamamoto, K. Kasuga, H. Omizu, M. Kumada, J . Organomet. Chem. 113 (1970) 127.
[99] H. Brunner, G. Riepl, Chem. Ber., in press.
[lo01 T. H. Johnson, K. C. Klein, S. Thomen, J . Mol. Cutal. 12 (1981) 37.
[loll H. Brunner, G. Riepl, Angew. Chem. 94 (1982) 369; Angew. Chem. I n f .
Ed. Engl. 21 (1982) 377; Angew. Chem. Suppl. 1982. 769.
[lo21 H. Brunner, G. Riepl, H. Weitzer, Angew. Chem. 95 (1983) 326; Angew.
Chem. Int. Ed. Engl. 22 (1983) 331; Angew. Chem. Suppl. 1983, 445.
Phosphorus Ylides in the Coordination Sphere of Transition Metals :
An Inventory
By Hubert Schmidbaur*
Dedicated to Professor Ernst Otto Fischer on the occasion of his 65th birthday
Phosphorus ylides are not only classical reagents in organic chemistry, but also play an increasingly important role as novel components in organometallic compounds. These metallic “ylide complexes” are either synthesized from “preformed ylides” and coordination
compounds by addition or substitution, on the building block principle, or they are formed,
in sometimes complicated reactions, from phosphanes, metal complexes, and C , substrates
in the coordination sphere of the metals. The resulting metal-carbon bonds are greatly modified in their properties by the immediate presence of the phosphonium center and often belong to the most stable of M-C structural units. The metal can come from any group of the
periodic table, including the lanthanoids and actinoids. Numerous preparative and structural studies are gradually enabling us to gain an overall picture of the scope of this area of
research.
1. Introduction
Phosphorus ylides are remarkable compounds from
many points of view. Too little emphasized, however, is the
fact that they can be classified as phosphane-stabilized
carbenes, and may thus constitute the oldest and best studied class of carbene complexes[’]. In contrast to such complexes with all the other elements these phosphorus-carbene complexes are very stable even in the absence of stabilizing substituents or salt components. This exceptional
position vividly manifests itself on comparison with the
carbene complexes of the homologous amines, arsanes or
stibanes (nitrogen, arsenic and antimony ylides) or of sulfanes (sulfur ylides), which decompose very easily and act
as carbene camers. Therefore, phosphorus ylides are not
only described as dipolar carbene complexes with the formula Ia, but also in the sense of a formal charge compensation as neutral molecules Ib12*31.Their reactions and
most of their physical properties, however, give a clear indication that their actual bonding pattern is much closer to
the 1,2-zwitterionic structure la. The functional effect of
the two reactive centers-carbanion and phosphonium
group-is, however, differently exhibited and the phosphonium unit remains very much in the background during the primary step of almost all the reactions.
[*] Prof. Dr. H. Schmidbaur
Anorganisch-chemisches Institut der Technischen UniversitBt Miinchen
Lichtenbergstrasse 4, D-8046 Garching (Germany)
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
The absolute dominance of the carbanion formed by polarization of the carbene function becomes obvious in
ylides and particularly in “salt-free’’ ylides from an extraordinarily high Brmsted basicity, which is of importance in the secondary process known as “transylidation”
in reactions with free ylidesL41.
II
m
R3P-CH3
R$-CH2
\Me
IU
The exceptional donor character with respect to practically all metals M (formula III), which in many cases suffices to drive almost all other ligands out of the metal’s
coordination sphere, is even more strikingr5].There are alternatives, however, if an ylide attack is also possible at
one of the ligands already present, whereupon the strong
nucleophilicity of the ylide comes into play, which also
constitutes the most important characteristic of ylide reactivity in organic chemistry“].
The three important characteristic features of the phosphorus ylides-basicity, donor activity, and nucleophilicity-are expressions of the excess charge on the ylide C
atom, which is not adequately compensated by the Pe center and which expresses itself differently depending on the
0 Verlag Chemie GmbH. 6940 Weinhelm, 1983
0570-0833/83/1212-0907 $ 02.50/0
907
nature of the reaction partner. From this it will be clear
that the enormous variety of oxidation levels, coordination
numbers, and complex geometries that characterize metals,
offers a plethora of interaction possibilities for ylides that
are only now beginning to be understood.
It must be emphasized here that-as can be seen from
formula 111-ylide complexes of metals principally constitute organometallic compounds, whose novel metal-carbon bonds lie under the electronic and steric influence of
the onium center. In contrast to syntheses with organometallic reagents (RLi, MNgX, etc.) the cation accompanying
the anion (Ia!) stays in the product (111) which is formed
after attachment to the metal, whereas otherwise the organic residue is separated from the LiQ or Mg2’ cations.
This rapidly expanding research area has received further impetus from new developments ranging from multifunctional ylides, on the one hand, to multinuclear metal
compounds on the other. The symmetrical ylides of type
IV bearing the trivial name “carbodiphosphoranes” play
an important role among the multifunctional ylides and
may be formulated as bisphosphane complexes of naked
carbon.
r
fB
R3P’
1
r
e
\PR3
20
2. Complexes of Phosphorus Yiides with Scandium,
Lanthanum, the Lanthanoids, and Actinoids
It is characteristic of the special position of ylide complexes amongst organometallic compounds that, even with
the elements of Group 3A where the development of the
chemistry of o-bonded organometallic compounds had a
late start, derivatives of methylenephosphoranes R3P=CH2
already play a significant role.
It was first shown in 19761g1that the elements La, Pr, Nd,
Sm, Gd, Ho, Er and Lu could form stable “homoleptic” ligand sheaths (2) with “difunctionalized (CH3)3PCH2”1’0.”1
that is with the corresponding anion [(CH3),P(CH2),l0.
The “mixed” halide complexes 1 are reaction intermediates. Complexes of type 1 and 2 are exceedingly sensitive to oxidation and hydrolysis, but some are stable to
200°C under an inert gas. Compounds of type 2 were precursors for the hexamethylmetalate anions 3, first described some five years later1”I:
-
R3PH
J
IJL
It should be noted that other similar element-carbon-elefB 2.0
fB
ment bridges E=C=E or E-C-E are, as yet, almost completely unknown, which once again emphasizes the special
position of the phosphorus double ylides.
The stimulus for working with ylide complexes results
from the real possibility of utilizing the compounds for
metal-catalyzed reactions and for other purposes. The ever
increasing use of tertiary phosphanes as additional ligands
in stoichiometric or catalytic reactions at transition metal
complexes makes it more likely that ylide complexes play a
part as intermediates or products in such processes. In fact
it has been known for a long time that phosphorus ylides
can be formed in the coordination spheres of metals, and
hence the directed introduction of preformed ylides is no
longer the only preparative route to these complexes.
The literature concerning the formation and reactions of
ylides at metal centers, which is rapidly becoming more
difficult to survey, has been reviewed several times from
various points of v i e ~ [ ’ ~ ~These
- ~ ] . valuable reviews will
now be supplemented here, in that developments of recent
years will be put into perspective with the basic discoveries
of the past. The subject can best be systematized according
to the periodic table, since the interested reader is usually
seeking element-specific information, which is most easily
made available to him via this approach. Newer results are
treated in more detail than earlier ones, which have already received greater a t t e n t i ~ n [ ’ . ~The
- ~ ~ subject
.
will deliberately be restricted in this article to transition metal
compounds. The corresponding compounds of phosphorus
ylides with main group metals will be described in a fur908
ther paper. A review article on the complex chemistry of
the sulfur ylides has been published recently[81.
1
Ln=La.Pr.Nd.Srn.Gd,Ho.Er.Lu
3e
H3C-Ln-CCH3
H3C’
3
‘CH3
It was also discovered in 1976[13]that the bis($-cyclopentadieny1)scandium residue could be combined with diphenylphosphoniobismethanide to form a stable organometallic compound: The pale yellow, crystalline complex
4 is formed from (C6H5)2P(CH3)CH,[’41
via the lithiated intermediate; like the diamagnetic members of the series
21’21it could be characterized NMR spectroscopically.
4
The sixfold tert-butyl-substituted homologue of 2,
Ln = Lu, that has recently been ~ynthesized“~],
has proven
even more suitable for spectroscopic investigation, and it
has been shown by means of temperature-dependent ’H-,
I3C- and 31P-NMRspectra that there is an oligomer equilibrium 2a + 2b + ... in solution.
Consequently, these compounds behave like the homoleptic ylide complexes of nickel[’61 or zirconium[’71, in
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
A),
(2.29(4)
suggesting the presence of multiple-bonding;
the U-C-P angle is 142(1)".
which the bidentate [RzP(CHz)2]e ligands can function
both as bridge-builders as well as chelaters.
A lutetium analogue of 4 is, in contrast, strictly monomeric in benzene solution (5, decomp. 156"C)"51.
The mixed yliddhalide or ylide/alkyl complexes 6
formed from (C5H5)2L~C1or (C,H,),LuR(THR
and
(C6H5)3PCH2in toluene are important examples of lanthanoid compounds with terminal monodentate-coordinated
methylenephosphorane. Hydrolysis of 6a leads to cleavage
of Lu-C and formation of methyl(tripheny1)phosphonium
c h l ~ r i d e " ~ . The
' ~ ~ .alkyl compounds 6b and 6c can also be
obtained
from
6a
with
alkylmetal
reagents.
Me3P=CHSiMe3 was used instead of Ph3P=CHZ[191.
The
hydride- and methyl-compounds 7a and 7b, until now
only accessible with pentamethylcyclopentadienyl ligands,
can also yield 1 :1 adducts (8a and 8b, respectively) with
(C,H,),PCH,, but they undergo intramolecular metalation
with expulsion of H2 or CH, and formation of the cyclic
ylide complex 9, even at 20"CL201.
7
a, A=H.
8
9
b, R=CH3
The reaction 8a, b + 9 is not without precedent, since
similar ortho-metalations also occur with platinum(o),
which-in accord with the oxidation state of the metalare classified as oxidative additions'"].
To date, the sole coordination partner for phosphorus
ylides amongst the actinoids is uranium. Depending
upon the stoichiometry of reaction, (q5-C5H5)3UC1
reacts with the lithiated ylides C&(CH3)2P=CH2 and
CH3(C6H5)2P=CH2to give a range of new organouranium
compounds, some of which exhibit remarkable types of
bonding. Thus, with a 1 :1 ratio of reactants["] the dark
green complexes 10a and 10b are formed, of which the
structure of 10a has been established by X-ray diffractionEZ3].
According to the X-ray structure analysis this compound possesses the shortest U-C bond known to date
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
With molar ratios of 1 :2, on the other hand, a red binuclear complex 11 is formed, presumably according to the
mechanism outlined below; a monoclinic modification
(containing 1/2 mol diethyl ether) and an orthorhombic
modification (containing 1 mol n-pentane) have been elucidated by X-ray diffraction. The latter has been identified
as (M)-[{(~-~-CH)P(C6H5)zCHZ}U(CsH5)2]2r
taking into account the configuration at the chiral ~ e n t e r s ' ~ Both
~'~~~.
ylide ligands in the dimers are effective as bridging and as
chelating agencies, whereby in each case one of the C
atoms carries out both functions.
Finally, with a molar ratio of 1 :3 a gold-colored, mononuclear organouranium compound 12 is obtained1'*'.
Aside from the remaining q5-cyclopentadienyl ligand,
12 contains three diphenylphosphoniobismethanide ions
as chelate forming ligands. The uranium atom is thus surrounded by eleven carbon atoms, of which six represent
ylidic centersL2'!
3. Complexes of Phosphorus Ylides with
Titanium, Zirconium, and Hafnium
The first attempts at synthesizing organo-titanium and
-zirconium compounds with ylidic ligands met with little
success, since the simplest reactions, e.g. of TiCla with
(CH3)3PCH2,only yield insoluble and hardly characterizable
Analytical data gave only rough indications of the stoichiometry.
However, novel prototypes of polynuclear ylide complexes were discovered on using dimethylamino- or methoxy-titanium halides as starting materials. Thus, reaction
909
of [(CH3)2N12TiC12with (CH3)3PCH2affords the 1,3-dititanacyclobutane 13[261,
whereas reaction with (CH30)3TiCl
yields the double octahedral species 14[271.
In 14 the Ti
atoms appear as substituents at the ylidic C atom-a situation which was previously only known in Si analogues'281.
The ylide braces of the coordination octahedron in 14 are
unique. In both reactions the ylides are deprotonating in
the sense of a transylidation, giving rise to the formation of
[(CH,),P]Cl as a byproduct.
HiC
-P<\
Me2
N
17
Without prelithiation of the ylides[26J,the double onium
salts 18 are formed,
2 Cle
Me2N
NMe2
\ /
180, M=Ti
lab, M=Zr
or, in the case of triphenylphosphoniomethanide when
M = Zr or Hf, the transylidation productlZ7J,19.
"2
ICSH,I,MCI~
+
2 PbPCH2
-
Ci
IPh3PCH3lCI
+
e/
(C5H5)2MT.
@
C'"PPh3
H
19a. M=Zrj m.p=199-210°Cyellow
19b,M=Hf, m.p=120°C(decomp.).yellow
A stimulating analogy to the bonding observed in the titanium compound 13 has recently been discovered in the
zirconium compound 15, which is formed from
[(CH3)3CCH2kZrand (CH3),PCH2 as yellow, monoclinic
crystals. The Zr-C bonds of the 1,3-dizirconiacyclobutane
are extremely short, whereas the PC2Zr chelate rings correspond to the known models with other transition metals.
The Zr-C bonds trans to the ylide bridges (equatorial) are,
however, significantly longer than the apical Zr-C
bonds.
X-ray structure analysis of 19a reveals that both of the
bonds in the Zr-CH-P bridge possess substantial multiple bond character: Zr-C 2.152(8), P-C 1.708(6) A,
Zr-C-P 135.9(3)". Thus, the bonding is similar to that in
the uranium compound 10a. Correspondingly, compounds
10a and 19a also behave similarly in their reactions, of
which the important CO insertion has been studied in detaip3. 281.
Yet another type of product is formed on reaction
of alkylbis(q5-cyclopentadieny1)zirconium hydride with
(CH3)3PCH2:elimination of alkane takes place, resulting
in formation of the green-colored ylide chelate 20, which
undergoes a hydride/chloride exchange with chloromethane to give the colorless derivative 21[29J.
Me
Me
Me3
Derivatives which have a close relationship with their
metal(II1) analogues have been prepared by the reaction of
bis(q5-cyclopentadienyl)metal halides of Group 4a with
phosphorus ylides. According to magnetic and ESR studies the simplest type 16 is to be regarded as a genuine
Ti"'(d') ~ o m p l e x ~ ' ~ ~ .
Thermal rearrangement of 21 at 65-70°C leads to the
yellow isomer 22, which corresponds to the triphenylphosphonio compound 19a. In the case of 19a, isomerization
to the ylide-chelate of type 21 is precluded, since no second alkyl group with an acid H atom is available at the P16a, R ' = H , RZ=R3=Ph; m.p.=152--153"C;
purple
purple
16c, R ' = H , R2=R3=Me; m.p.= 122--123°C; black-green
16d, R'=Me, R Z = R 3 = E t ; m.p.=61--63"C; dark green
16b, R'=H, R2=Ph, R3=Me; m.p.=147-15OoC;
A related double ylide complex 17 exhibits similar properties[261.
910
An electron-rich bis(q5-cyclopentadienyl)zirconium(~~)
complex (d2!) reacts with (C6H5)3PCH2with transfer of
methylene to the metal atom. The likely intermediate 23,
containing a Zr-CH2-P(C6H,)3 moiety, is converted by
elimination of P(C6H5)3 into the-albeit nonisolablecomplex 24[30,311.
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
' 'L
23
2.t
from the C atom and transformed into an 0x0 bridge. The
overall process is one of the most remarkable findings yet
in the field of ylide complexes. The molecular structure of
28 in the yellow, monoclinic crystal has been determined
X-ray crystallographically. The P-C separation is
1.751(19) and, thus, can scarcely be classified as ylidic.
The unequal Ta-C distances, on the other hand, are relatively short (2.187(17) and 2.354(17) A)[371.
The nonisolable intermediate formed in the reaction of
the electron-rich Ta"' complex (C5H5)2TaCH3[P(CH3)3]
with (CH3)3PCH2 should be formulated as an ylide adduct; elimination of phosphane, even under mild conditions, then furnishes the complex 29, which reacts with
further (CH3),PCH2 in a similar manner to yield the ethylene complex W3*].
A
Reaction of an acetyl(chloro)titanium(Iv) complex with
(C&)3PCH2 does not lead to the formation of an ylide
complex or to the product of a simple Wittig olefination,
but to the formation of a metal ketene complex[321.
CI
/
(C5H5/z T i
+
Ph3PCH2
\
-
0
(C,H512Ti//
+
Ph3PCHTCle
C'
\
//C-Me
0
cH2
Phosphonio-cyclopentadienide and -cycloheptatrienide
complexes of titanium, zirconium and hafnium (25, 26) involve a special relationship between metal and ligand and
will, therefore, not be considered here. None of the structures is as yet sufficiently well k n ~ w n ' ~ ~ . ~ ~ ] .
25, M=Ti,Zr.Hf
Ph3F-O
26
30
e /
lC5 H5)2Ta
29
\
CH2
I e
ti2C-PMe3
4. Complexes of Phosphorus Ylides with
Vanadium and Tantalum
Vanadium and its homologues have as yet rarely been
used as coordination centers for phosphorus ylides. True,
the cation formulated as [(C6H5),PCH2I4V2@
constitutes
one of the oldest examples of a transition metal-ylide comp l e ~ ' ~ yet
~ ' , no other such vanadium compound has since
been described, apart from the chelate 27. The complex 27
can be synthesized according to the method given for 4 or
16a and has been unequivocally identified as a V"'(d2)
complex by ESR spectro~copy"~~.
Noteworthy here, is the ease of transfer of the carbene
from the ylide to the metal, i. e. the ylide multiple bond (!)
can be ruptured without difficulty. Closely related tungsten compounds exhibit similar behavior (cf. Section 5.3).
The reaction steps formulated above for (CH3)3PCH2
can also be accomplished with (CH3)3P=CHC6H5r
(C6H5)3P=CHC6H5 and (C2H5)3P=CHCH3, but not with
(C6H5)3P=CH-C(CH3)3 and (C2H,)3P=CH-C(0)CH3,
where limits are set by steric hindrance or reduction in nuc l e o p h i l i ~ i t y ~In~ contrast,
~~.
in the reaction
+PMe3
Clz1C5H5)Ta=CH-C(CH3)3
+Ph3PCH2
-fPh3PCH,ICI
CI(C,H5)Ta=C-C(CH3),
\
PMe3
the ylide functions only as dehydrohalogenating base[391.
5. Complexes of Phosphorus Ylides with
Chromium, Molybdenum, and Tungsten
Surprisingly, a binuclear tantalum complex 28 was obtained on reaction of a dihydride precursor with CO followed by P(CH3)3[361.
The [(CH3)3Pm-CH]unit, which functions as a bridge ligand in 28, is evidently formed in the course of this reaction from complexed CO, whereby oxygen is separated
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
5.1. Homoleptic Chromium- and Molybdenum-Ylide
Complexes
The homoleptic tris-chelate complex 31 was first isolated almost exactly ten years ago as a red, sublimable,
crystalline substance. It represents the first phosphorus
ylide derivative of trivalent chromium. The preparative
methods used became models for the later synthesis of further analogous trivalent complexes of the earlier transition
metals, which have already been mentioned.
Compound 31 is strongly paramagnetic (peff
= 3.6pe),
which is in good agreement with the d3 state of the central
atom, chromium(iIi)f401.
91 1
CrCI3.3THF +3 lMe2PlCH2)21Li
-3LiCI
-3THF
Me2Ps
z /sPMe2
\e,d
/g\
Li3CrlC6y)6
+
3 IMe,PICI
/
-6C6H6
-3LiCI
Me2
31
A red intermediate product 32, in which the metal carries a further phenyl group, can also be isolated in the synthesis of 31 from Li3Cr(C6H& The reaction of 31 with
three equivalents of t01an[~*~
leads, with opening of the
chelate rings, to formation of the yellow trisalkynyl complex 33.
32
pounds. They are accessible via various, widely differing
routes and have been the subject of intensive research for
more than 20 years, not least because of their close relationship to the corresponding carbene complexes. The extensive findings are classified in this review article according to the methods of preparation.
5.2.1. Direct Coordination of Hide Ligands at the Metal
Whereas hexacarbonylchromium does not react with
"salt-free" phosphorus ylides only by CO substitution, but
rather by attack at the CO ligand, its tetrahydrofuran derivative (C0)5Cr.THF is a suitably modified, selectively
reacting starting compound for the introduction of ylides
at the metal atom[481.
33
When the educts of the synthesis of 31 are allowed to
react with ylides of the type (C6HJ3PCH2 rather than with
metalated or metalatable alkyl-substituted ylides of the
type (CH3)3PCH2, then ortho-metalation of the phenyl
rings occurs to give the ylide complex MIa]. Later, analogies for this mode of reaction were found, inter alia, for platinum and lutetium[20,2'1.
Chromium(I1) and molybdenum(m) halides react with
(CH3)2P(CH2)2Lito yield binuclear complexes 35a and
35b, respectively, which have attracted continual attentionr42*431.
X-ray diffraction s t ~ d i e s ' ~have
* ~ ' ~revealed extremely short metal-metal distances (MEM) in these diamagnetic compounds.
The particularly simple ylide ligands, whose coordinating CH2 groups do not lead one to expect either backbonding phenomena or other electronic or steric side effects, are almost ideal for further experimental investigation of the M=M unit[461and for its theoretical descriptionI4'1.
35a, b can be crudely described as having paddle-wheel
structures, with a metal axis of 1.895(3) (CrrCr) or
2.082(2) A (MogMo). The metal carbon distances are
2.21-2.23 (Cr-C) and 2.30-2.32
(Mo-C). It is possible to distinguish long P-C methyl distances (1.811.90
and short P-CH, distances (1.77-1.79 A) at the
quasi-tetrahedral P atom, thus indicating residual ylide
character for the latter[",451.
Benzene(dicarbony1)chromium-tetrahydrofuran reacts
analogously and yields complexes 37,which have been described in seven different R,R-c~mbinations~~*~.
The halogen atoms of the complex salts [(C2Hs)4N~m[(CO)sCrBr]e
are also good leaving groups, which facilitate the preparation of ylide complexes. Amongst the known examples
(> 15), there are also some with phosphane ligands at the
metal["].
R
+Ph3P=CRR
e I m
INR~I%CO)5CrBrle -NR: Bre lCO)5Cr-C-PPh3
I
36
R'
Ligand exchange also takes place in the triphenylphosphane derivative :
IC 015 Cr IPPh, )
p"p
+Ph P'CHz
h3
e
s
1CO),CrCH2PPh3
Reports that Mo(CO), reacts with (C6Hs)3P=CHCH3 to
form a complex (CO)3Mo[(C6H5)3P=CHCH3]3have not
been confirmedcs2].The reaction of ylides with conjugated
olefinic double bonds can lead to multiple CO exchange,
whereupon n-ally1 type complexes 38 can be f ~ r m e d [ ~ ~ - ~ ~ ] :
m
A
A)
5.2. Carbonylmetal Complexes of Phosphorus Ylides
Carbonylmetal(o) complexes of phosphorus ylides count
among the most investigated of the metallic ylide com912
Hd
\\
M=Cr.Mo.W
CHZ
38
With phosphoniocyclopentadienide, $-CSH4=PR3 coordination is a ~ h i e v e d [ ~ ~which,
- ~ ~ I ,because of the special
constitution of the product (cf. also 25) will not be dealt
with here. h'-Phosphabenzene complexes are also not discussedr6']. A (CO),W complex of Ph3P=C(SMe), was
Angew. Chern. Int. Ed. Engl. 22 (1983) 907-927
found to be S-bonded[6'1. On reaction of phosphanes with
the tropylium(tricarbony1)metal cation of chromium[621
(but not of molybdenum1631),related q5-organometallic
compounds 39 with a phosphonium center in the side
chain are formed.
ables access to a series of ylide complexes, in as far as their
substituent combinations at the carbene C atom are realizable. This applies in particular to phosphorus ylides with
electron-rich substituents.
M=Cr.W (rarely Mo)
Fluorene ylides with additional phosphane function
yield spirocyclic complexes 40, but also, e. g., the $-fluorene complex 411a1.
5.2.2. Sulphane/Phosphane Exchange at Surfr Hide
Complexes
In the synthetic methods already discussed, an intact
phosphorus ylide was coordinated to the metal atom. Important alternatives to this are the formation of the ylide
within the coordination sphere of the metal, either from
another ylide complex by onium-center exchange or (cf.
Section 5.2.4) from a carbene complex by addition of a
phosphane. The first method has been used to advantage
on (CO)5Cr complexes of the dimethyl(oxo)sulfoniomethanide (CH3)2S(0)CH2, whereby dimethyl sulfoxide
proved to be an excellent leaving group. A particular advantage of this method is that unstable ylides can be fixed
to the metal and stabilized[651.
Arsanes undergo similar reactions[&!
The reversibility of this equilibrium has been demonstrated kinetically[781.Typical enthalpies of reaction lie in
the range -15 to -20 kcal mol-'. The rates of reaction
are in the order Mo > W > Cr1791.
The reactions do not, however, always proceed according to the desired scheme. Apart from the CO exchange already mentioned, a strong substituent effect is also observed. Thus, (C0)5W=C(C6H5)Z reacts with P(CH,), in
the expected manneF4], whereas reaction with P(C6H5),
leads to an unexpected cleavage of the ylide
(C6H5)3P=C(C6H5)2from the
The last mentioned
reaction is one of the few examples known where the intact
ylide can be removed from the metal.
Mides with P-H bonds can undergo rearrangement to
phosphane complexes[711:
FSMe,
ICO)5&-CH2
-Me,SO
-
e
(CO)5Cr-CH2
/
KO),W=C,
/PR,
36
5.2.3. H& Complexesfrom Haloalkyltungsten Complexes
(CSH,)(CO),WCH2C1, a (chloromethy1)tungsten complex which is now readily accessible, can easily be converted into the corresponding ylide derivative by reaction
with P(C&)3 in boiling methanol[671.Analogous substitution reactions have been demonstrated with Fe, Rh, Ru,
etc. (cf. Sections 7 and 8).
-
Ph
Ph
Ph3PCH2
(COISW+PPh3
+
H C-
/
-\
OMe
OMe
5.2.5. Reaction of Carbyne Complexes with Phosphanes
Phosphanes can add to metal carbynes analogously to
their reaction with metal carbenes. Metal-substituted phosphorus ylides 42 are formed whose substituents depend on
the
If the central atom is tungsten, an addiRPR3
BrIC0)LCrEC-R'
PPh,
C5HSlCO)3WCH2CI
\
Me
With ylides (instead of phosphanes), carbene complexes
usually undergo Wittig olefination, whereby the (CO),W
fragment formally plays the role of the oxygen atom of the
aldehyde or ketone (!)["I:
Q
+PR,
,OMe
lCO15Cr-P-CH
I
Ph
Ph
0
Ye
OMe
e
I Q
(CO)SCr-C-PMe,H
I
PR3-
BrlCO),,Cr-C
\
12
R'
IC5H5(CO13WCH~~Ph31Cle
13
5.2.4. Hide Complexesfrom Stable Carbene Complexes
and Phosphanes
Although the first investigations had shown that the
thermal reactions of (CO)5Cr carbene complexes with
phosphanes and other nucleophiles finally lead to irreversible CO e x ~ h a n g e [ ~ ~
later
? ~ ~studies
],
showed that, under
mild conditions, the reversible addition at the carbene carbon atom predominate~[~'-~~].
This important reaction enAngew. Chem. Int. Ed. Engl. 22 (1983) 907-927
X=CI.Br
R=Me
R':Ph.C6HLMe
14
913
tional partial CO exchange takes place and, in certain circumstances, a cleavage of the metal carbon bond[S71;in this
case semi-ylide salts 44 can be isolated.
The binuclear carbyne complexes 45 add P(CH,), to
form an ylide-bridged product 46 whose constitution has
L9
50
51
been determined by X-ray diffraction a n a l y s i ~ [ ~This
~*~~~.
reaction, which until now is without parallel, demonstrates
The binuciear tungsten complex 52 with q1,q2-bonded
that in suitable cases the bridging double coordination of
3,3-dimethylated
ally1 groups adds phosphanes at C-3 to
an ylide to a metal is preferred over the terminal coordinagive
a
product
53
with a op-bonded bridge ligand1961.
tion (as in 42 or 43).
OC\
/co
(CO)SRe-M=C-Ph
/ \
oc
-
CO
b
Ph
PMe3
V
PMe3
'C/
/ \e
(CO),Re-M(CO),
\C/
45
M=Cr.W
1
L6
5.3. Ylide Complexes from
Activated Alkyltungsten Compounds
52
53
The structure of one representative of the range of compounds formed from five different phosphanes was determined by X-ray diffraction. Because of its constitution, the
bridge ligand in 53 can no longer be regarded as a phosphorus ~ l i d e ' ~ ~ ] .
Surprisingly, it has been found that a terminal ylide
complex [(Me3P),(CO)C1WCH2PMe3]@
is formed on reaction of the cationic carbene complex [(Me3P)4CIW=CH2}b
with CO. The reaction has been interpreted as a W+C
phosphane shift[971.
In the presence of donor molecules, alkyl compounds of
bis(cyclopentadieny1)tungsten exhibit a complicated reaction behavior, the unraveling of which is of considerable
interest for an understanding of the metal catalyzed reactions of olefins. Trapping the reactive intermediates with
phosphanes to give ylide complexes has allowed great progress to be made in the study of this p r ~ b i e m [ ~ ~ - ~ ~ ~ .
The ylide complexes 47 and 48 can be isolated with
P(CH,), from the equilibria between metal hydride, alkyl5.4. Ylide Reactions at the Ligands of
metal compound, and metal-olefin complex, if suitable
Carbonyl Complexes
stoichiometric conditions are chosen:
The finding that strongly nucleophilic phosphorus ylides
attack the metal complexes M(C0)6 at the CO ligand'98,991
could be explained step for step as a transylidation reaction['OO1.The following scheme illustrates the reaction
which has been confirmed for several cases (including also
metals other than Cr, Mo, W):
Similar products are formed when carbene precursors
or via radical sources
are produced
and proton acceptor^[^^,^^^ from (CSH5)2W(CH3)2and
(C,H,)2W(CH3)C2H,. The PR, components employed
were P(CH,),, P(CH3)2C6H5and P(C6H5)zCH3.Both the
kinetics and equilibrium of the reaction depend on the nature of the p h ~ s p h a n e ' ~ ~ !
A similar equilibrium between the phosphanes P(CH,),
and P(CH3)2C6HSwith the complex cation 49, in which
one of the cyclopentadienyl rings is additionally attached
to the tungsten atom by a C2 bridge, affords an ylide complex 50 which only slowly reacts to form the stable end
product 51[951.
914
The zwitterionic carbene complex 54 is transformed
with a second equivalent of ylide in a transylidation step
into the corresponding anion, which can then be derivatized by 0-alkylation to the neutral carbene complex 55
bearing ylide substituents['OO1.
Such a derivatization can
also be accomplished with a powerful silylating agent
(RX = Me3SiOS02CF3). An X-ray structure anaiysis has
been performed on the
Related complexes can also be synthesized directly from
the hexacarbonylmetal with silylated ylides"021.An arsenic
ylide corresponding to 56 is also known.
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
UV irradiation of W(CO)6 and the double ylide
Ph3P=C=PPh3 yields the acetylide 57, which crystallizes
as yellow needles (m. p. 140-142°C)[991.
570
(COISW=C=C=PPh,
WICOI,
Ph,P--C-PPh,
-
t
-Ph3PO
57b
(C0)5eW-CEC-PPh3
The reaction can be classified, on the basis of the mesomeric form 57a, as a Wittig olefination of CO in the
coordination sphere of the meta1r99*’oo1.
The simple (CO),W-adduct of hexaphenyl carbodiphosphorane obtainable from (CO),W-THF is easily hydrolyzed and, on reaction with water, yields adduct 58 with
0x0-coordination ; the structure of 58 has been confirmed
by X-ray diffraction[’031.
tainable in good yields by the reaction of alkali-metal salts
of the diphosphonio triple ylide 60 with the tetrahydrofuran adduct of MnBr$1071.
The metal atom of the paramagnetic, brilliant yellow
product (pert
= 5.9pB, dS) is pseudotetrahedrally surrounded by four ylidic carbon atoms. Both the remaining
carbanion centers (P-CH@-P) point away from the metal
atom. The X-ray structure analysis was carried out on a
bistetrahydrofuran adduct, whose solvate molecules are
likewise not coordinated to the metal[’071.
The manganese(1r) complex 63 formed on reaction of
the lithiated boranato-bisylide 62 with MnCl,. THF also
contains pseudotetrahedrally oriented ylidic C atoms. The
yellow compound, which may be regarded as a simple tetraalkylmanganese, is paramagnetic (perf.
= 5.84pB, d’).
Crystal structure analysis revealed Mn-C distances of
2.214(4)-2.239(13)
which correspond to a carbon-manganese a-bond. The P-CH2 and P-CH3 distances are CQ.
1.75 and 1.82
respectively, as are typical for other ylide
complexes.
A,
A,
H2
The reaction of a chloro(dipheny1)phosphane complex
with ylides yields a related P-coordinated ylide complex
59“041.
ICOISW-PP~~CI
2P h,PCH,
\CH
//
-Ph3PCH3@Cle
Both six-membered rings in 63 are in a chair conformation, the boron and phosphorus atoms have tetrahedral environments[’08’.
A wide variety of dicarbonyl(methylcyc1opentadieny1)manganese complexes 64 are accessible from the THF
precursor and free ylides (cf. also Section 5.2.1).
59
Ph3P
6L:R’:H.
Metal complexes with arsane ligands, on the other hand,
are cleaved by ylides at the M-As bond[’051.
Reaction of (C6H5)$CH2 with the binuclear complex
(C5H5)ZMoz(C0)5
yields a whole range of products, whose
formation is still not completely clarified[’061.
R3=Ph3, Ph2Me. PhMe2, Me3,
R’=Ph, R3=Ph2Me. PhMe2. Me3
The analogous tricarbonyl compounds, on the other
hand, react with ylides at the CO ligands according to the
scheme described for Cr(C0)6 (cf. Section 5.4). The following scheme shows the reaction paths and products discovered up to now[’091.
6. Complexes of Phosphorus Ylides with
Manganese and Rhenium
LIC013Mn
Oe
Me3PCH2
L(C0I2Mn=C
No simple coordination compounds of ylides with manganese have as yet been described. Only specific multiple
ylides form homoleptic manganese complexes with phosphonium centers to the metal atom. The best characterized of these are the bisbenzodiphosphepinyl complex 61
and the boron-containing bis-chelate 63.The former is ob-
/
\
MeLPe
C=PMe3
H
MeOS02F
OMe
L(C012Mn=C
\
:=PMe3
n
~M~OSO~F
L (CO12Mn=C /OMe
\
C=PMe3
65
Me,
-
+Me3PC H2
-MeLP@S03Fe
SO,F~
L(C012Mn=C ,OMe
\
e
CH-PMe,
Me‘
L = CH3- C5Hk
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
915
The structure of the end product 65 has been confirmed
by X-ray diffractiod1lo1.
Surprisingly, the course of the reaction with the ylide
(C6H5)3P=CH2is much more complicated. The complex
66 is one of the numerous compounds isolated from the
product mixturelll'J.
Q@
PFse
72, M=Mn.Re
Addition of P(C6H& to the alkyne complex 73 occurs at
the *C atom, which in the product is no longer attached to
the metal atom11151,
so that the compound 74 contains an
ylide function in the fi-position. X-ray structure analysis
confirms the P=C-bonding :
66
0
Bromo(pentacarbony1)manganese and the analogous
rhenium compound react with (C6H5)3P=CH2(with transylidation, cf. Sections 2 and 3) to give the metal substituted
ylide 67L'001:
+R,P=C=PR,
-R,PO
-
L=CH~-CSH,
M=Mn. Re
Pseudo-Wittig olefination and phosphane oxide cleavage occur once again with the double ylide and the same
starting complexes['001(cf. Section 5.4). The structure of the
manganese compound 68,which can also be formulated as
an acetylide, has been confirmed by X-ray diffraction"". 1121
A carbyne complex of manganese also allows single and
double addition of tertiary phosphane, according to the
scheme described for the 6a-elements Cr, Mo, and W (Section 5.2.5). The product 69 then undergoes a cleavage of
the Mn-C bond to yield (R3P)2CR@XQ
salts[871.
e
PD..
C5H5(C0126n-C-Ph
PR
C5H5(CO12Mn=C
/'
'"
PR3
\
1
BFLe
ePR,
e
l
C5H5(C0l2Mn-Y-Ph
BF,~
Med
7L
73
L(C0)2Mn-N=C-Cs
68
BrlCOi,M=C=C=PR3
c=o
I
COOMe
The attachment of C6H5(CH3)2P=CH2to a nitrile complex leads to formation of the adduct 75, whose structure
was deduced from spectroscopic data
67
(COIsMBr
PPh3
(CO13M
@+R3
69
HLMe
-
MeH, C,
\
R3PCH2
L(C0)2Mn-N
H
/"-"\
H
75
PR3
The ylide complexes 46 with Cr-Re and W-Re bonding have already been mentioned in Section 5.2.5185,86J.
7. Complexes of Phosphorus Ylides with
Iron, Ruthenium, and Osmium
The elements of the eighth subgroup of the periodic system often reflect the synthetic principles and compound
types already found with manganese and rhenium.
Homoleptic ylide complexes of iron only exist in the
presence of complex ligands, such as have been described
for the manganese complexes 61 and 63 (cf. Section 6).
The Fe" analogues 76 and 77 are tetrahedral, high-spin
complexes whose metal atoms coordinate exclusively aliphatic carbon
log].
Wide complexes which contain other ligands aside from
the ylide are formed from reactive CSHS(C0)2FeL@
precursors (L = tetrahydrof~ran)'"~'or the corresponding halides["s1 by reaction with free ylides, or from the chloromethy1 compounds with p h ~ s p h a n e ' A
~ ~range
~ . of structurally
ill-defined adducts is formed on reaction of
(C6H5)3P=C5H4with FeCl2['l9I.
An ylide complex of rhenium 71 is obtainable by addition of phosphane to a cationic methylene complex 70; the
nitrosyl ligand is not attacked11131.
Unactivated C5H,(C0)3Fe@, on the other hand, reacts
(like Fe(CO)s198J),with ylides with attack at the CO ligands; transylidation yields the compounds 79 and
80 1120.1211
Triphenylphosphoniocyclopentadienide can be readily
incorporated as a ligand in the complex 72I"'I.
916
The complex 79 (R=CH3) has been subjected to an XOf great interest are the novel
ray structure
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
0
+2 R3PCH2
CsH51C012Fe-C //
\
C5HSlC013FemXe
79
C=PR,
H
11 R'X
2) Base
-R,P=C(SiMe&
-HX
KO13
0
CsH51COl,Fe-C
'
80
\
/C=PR3
R'
rearrangements 80a +80b and the exchange of complete
ylide units 80c-+80d. Plausible mechanisms have been
suggested for both processesr1221.
-
0
CsHslC012Fe-C
//
h
\
C5H5iCO12Fe-C
Yo
Ph3PCH2
\
H/C=PMe2Et
Me
;;4
C5H5ICOI2Fe??Ph3
89
ICSH5(CO12FelH2C=CHRllmX
aob
80a
C5H51COlZFe-C,
can also be obtained from olefin complexes 88 by nucleophilic attack of tertiary phosphanes at one of the two C
atoms, which then carry the phosphonium center at the pC atom"271;they are stable when R = H, but very unstable
when R=C6H5[1281.Derivatives of type 92 are obtainable
from the related allene complexes 9111281,
while carbene
complexes such as 93 yield "genuine" ylide (alkylidene)
compounds 94[1291.
The latter are also accessible via the
acetylides and pho~phanes"~~'.
//O
+Me3PCH2
-
-Et3P=CHMe
C5HsIC0)2Fe-C
/C=PEt3
88
90
C-PMe3
IC5HsIC0)2FelH2C=C=CH2)lmXe
/
Me
80c
C5H51CO)2Fe~'Ph3
Yo
\
PPh3
C5 H51CO)2Fe-C
ICsH5(C0)2Fe=C=CH21mS03Fe
C5H5f C0)2Fe-C
PR3,HX
-
C5%1C0)2Fe-C
95
96
OSiMe3
//
CHPh
Xe
\m
PR3
The structures of the compounds 94 and 96 have been
determined by X-ray crystallography; the P-C distances
are found to be 1.811 and 1.800 respectively, and are
thus not ylidic in n a t ~ r e [ " ~ , ' ~ ~ ~ .
A comparable binuclear acetylide 97 yields a related
complex 98 on reaction with dicyc10hexylphosphane~~~'-
A,
83
(CO)LFe=C'\
C=PR3
H
The binuclear complex [C5H5Fe(C0)2]2 reacts with
(C6H5)3P=CHR' to give a mixture of products from which,
inreer uliu, 84-86 have been
1331
Ph2
97
8L
S03Fe
82
81
/
R,P=CHSiMe3
'
\m
PR3
94
CsH5(C0)2Fe-C=CPh
I
X'
m
C-PPh,
CH2
PR3
93
1
CH2
92
Mides and Fe(CO)s yield a range of analogous products
gl-~31100.102.1031
FelCO15
I
//
\
80d
Xe
R
91
H
Xe
R
85
Ph2
98
86
Reaction of Fe2(CO)9with Ph3P=C=PPh3 is reported to
lead to formation of (CO)3Fe(p-CO)[p-C(PPh3)2]Fe(CO)3,
but its structure has not been c ~ n f i r m e d ~ ' ~ ~ ~ .
The product 87, which unexpectedly carries a formyl residue at a formally ylidic C-atom (confirmed by X-ray diffraction), is formed in the reaction of Fe3(CO)Iz with
(C6H5)3P=C(SnMe3)2,a reaction which is extremely puzzlingI125. 1261-
Polyhuptoolefin-iron complexes react with phosphanes
to give a range of phosphonium salts with n-bonded metal
atoms which, on account of their difficultly defineable
composition, are only mentioned in p a ~ s i n g ~ ' ~ ~ - ' ~ ' ] .
In the organoiron compound 100 the benzocyclobutyl
residue is only q'-bound; the precursor is the cationic carbene complex 9911381.
The cationic ethylene complex 88 adds Ph3P=CH2 at
one of the two olefinic C-atoms to form a y-phosphonioalkylmetal compound 8911171.
A series of alkyl derivatives 90
Angew. Chem. Inr. Ed. Engl. 22 (1983) 907-927
917
Reaction and transylidation of (CO),Fe(PPh,CI) with
Ph3P=CH2 yields the complex (CO),Fe(PPh,-CH=PPh,)
with P-coordinated ylide['"''l.
Ruthenium complexes of phosphorus ylides are so far
limited to only a few examples. The complex 101['391
contains the chelate-forming ligand (CH,),P(CH2Xe.
IMe3PILRuCI2
H2C2PMe2
I
Me3PCHz
\
k3,CH2
Me3P,
101
?
M
2
eieI '
Me3P
'C
H2
This octahedral complex additionally contains the CH2depleted fragment (CH3),PCH?, which is formed by deprotonating a P(CH3), ligand. The reaction of Ph,P=CH2
with the binuclear ruthenium complex 102 surprisingly
leads to a compound with a CH2-bridge 103 and is thus
reminiscent of the behavior of [(C5H5)2Fe(CO)4]211"1;
the
details of the reaction are still unclear['401.
respond to the Mn" analogues 61 and 63. The crystals of
the paramagnetic d7-compound 109 ('perf
= 4 . 2 ~ are
~ ) isomorphous with those of the manganese complex 63. The
molecules thus have Co"-centers surrounded tetrahedrally
by C atoms.
Terminal monodentate (CH3)3P=CH2is a component of
the ligand sphere of a deep blue compound with the formula [(CH3)3P=CH2]zCoC12,110, whose exact structure,
however, is not known. The substance, which is insoluble
in all the usual solvents, is strongly paramagnetic and is
formed in the course of the oxidative addition of CH2C12
to (Me3P),Co, where intermediates with Co-CH2CI units
are probably f ~ r m e d " ~ - ' An
~ ~ alternative
].
synthetic possibility is the
reaction
of (Me,P),CoCI
with
(CH,)3P=CH,['461.
3(Me3P)&o
+ 2CH2CI2
----*
2(Me,P),CoCI
+ 2Me3P=CH,
Ph
0
0
103
102
-
+
(Me3PCH2)2CaC12 2(Me3P),CoC1 + 4PMe3
(Me3PCH2)2CoC12+ (Me,P),Co
110
+ 2 PMe,
Chelate forming (CH3),P(CHzhe units are realized in the
diamagnetic Co"'-complex 111, which is formed in a
transylidation reaction['41; its structure has been determined by X-ray diffraction['471.
Me
Cationic ethylene complexes such as 104 add tertiary alkylphosphanes to give y-phosphonioalkylruthenium compounds 105, which are related to the iron complexes 90 or
92 I1411-
H2f
PFse
PF6'
104
105
A RuC1, complex of (C6H5),P=C(CH), of unknown
structure requires further investigation"'91.
Osmium complexes of phosphorus ylides in the strictest
sense of the phrase are unknown. The products formed on
reaction of tertiary phosphanes with trinuclear osmium
clusters such as 106, which contain phosphonium centers
in the y-position to the metal (107), are of peripheral interestl14Z.1431
to,,
PR3
ICO
C-PR3
U"
107
'''
8. Complexes of Phosphorus Ylides with
Cobalt, Rhodium, and Iridium
Homoleptic ylide complexes of cobalt (as in the case of
Mn and Fe) are only obtainable with the triple ylide anions
of 60 and 62. These compounds, 108"071
and 1091'081,
cor918
L=cod/Z,CO. PMe3
R=Me.t Eu
LZRhe
\El
eRhL2
113
KO,,
IC 0 l3
106
Square planar rhodium(1) complexes contain the same
chelate formers in various ligand combinations. Starting
from 1,5-cyclooctadiene- or dicarbonyl-rhodium chloride
the mononuclear complexes 112 are easily obtained via
transylidation with free ylides and can dimerize to eightmembered metallocycles if the spatial requirements of the
substituents allow it: 1131148-1501.
When L = C O and R=CH, the equilibrium (in toluene
at 25 "C) lies on the side of the dimer with AH= - 10.5
kcal/m01"~~~.
Analogous complexes have been obtained
with the cyclic ylides (CHZ)nP(CH3)=CH2(n=4, 5) and
the double ylide (CH3)3P=N-P(CH3)2=CH2[1491.
Some of
the compounds are good hydrogenation catalysts"481. The
corresponding indium compounds are known but unfavorable solubility behavior and difficulties in crystallizing
them have, until now, hindered their satisfactory characterization""].
Some mononuclear cationic rhodium(n1) complexes
with monodentate ylides are accessible uia reaction of tertiary phosphanes with halomethyl precursors[1521,
e. g.
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
-
(codXNi + 4Ph3P=CHz
PMe,
[CSHS(Me3P)ZRhCHZI]~Ie
70PC
-2cod
(PhzPCHZPh)4Ni
[C5HS(Me3P)zRhCH2PMe3]ze2
I"
The corresponding cyclic variants 114 are formed with bifunctional p h ~ s p h a n e s [ ' ~ ~ - " ~ ~ .
It was possible to develop this process into a catalytic ylide
isomerization"601.However, it has so far proven impossible
to isolate a tetrakis(ylide)nickel(o) complex. Only in the
presence of accessory ligands is it possible to fix monodentate ylides on Nio.
Reaction of (cod)ZNiowith a C-benzoyl stabilized ylide
in the presence of PPh3 leads to oxidative addition of a
phenyl group to the metal atom with formation of an arylnickel(I1) complex 119['6*1.
Hexaphenylcarbodiphosphorane reacts with C O ~ ( C O ) ~
to give an ionic product [(CO)4Co-C(PPh3)z]"[Co(CO)4]0,
Phl
but no details of the reaction have been q u ~ t e d [ ~A~ ~ ' ~ ~ ~ .
+PPh3
Ph, NI
119
(codlzNi Ph,P=CH-CO-Ph
similar coordination unit is found in the indium complex
-2cod Ph3Pf \o/c'ph
115, which, however, decomposes in THF by intramolecular proton transfer and elimination to give the 1,8-cyclooctadienyl complex 116['551.
It has been postulated that 119 is one of the catalytically
active species in the oligomerization of ethylene['6z1.
PPh.
In contrast, it is possible to realize a homoleptic ligand
' a3
Ph,P=C=PPh,
6
(c0d1lr~PF~~
[codllr-C1e
PFse
115
sheath
at the Ni" center with phosphonium bis-ylide anb
ions. Reaction of nickel(r1) halides with Me3P=CHz or its
PPh3
homologues leads via intermediates with mixed ligands to
transylidation and formation of the mono or binuclear
ylide complexes 120a, b"6,147*1631.
-
1
7.5
-
+
,
I
//'HZ
117
RzP\
116
CH?
Reaction of [(c~d)IrCl]~
with (tert-Bu)zMeP=CHz leads
X,
Nix2
PMe,
e
,-PR2Me
Ni
7 \-FR2Me
xe
A
- v
v
R2Pe
Ylide
eN~e
4 P R 2
-PMe3
-HX
Me,P
I
R2=Mez.fBu2.(CH2IL.1CH2),
via transylidation to formation of the mononuclear ylide
120a
complex 117['561.
C O ~ ( C O likewise
)~
reacts with Ph,P=C(CH), with partial CO-exchange to give an ionic product 118a, whose
crystal structure has been determined"57'.
120c
tOICO1,
khlcodl
1180
118b
Triphenylphosphoniocyclopentadienide complexes of
rhodium, such as 118b, have a related structure['s81; instead of 1,s-cyclooctadiene (cod), they can also contain
C7H8,CO/PR3, C,Me5, etc.['"]. An ylide-adduct of RhNO,
is also known"'91.
lZOb
Formation of 120a is favored by bulky substituents R. A
polycyclic variant 119c is accessible after previous metalation of a spirophosphonium salt['641.X-ray structure analyses have been carried out on 119b and 1 1 9 ~ [ ' ~ 'The
~'~~.
corresponding Pd" and Pt" compounds were obtained by
analogous reaction^['^^.'^'^, e. g. according to
e
(Me3PI2PdCI2
Me3PCH2
MeZPe
rPM
C,e Yl,de
ePde
\-;Me3
9. Complexes of Phosphorus Ylides with
Nickel, Palladium, and Platinum
Me,
ePde ePMe2
9.1. Homoleptic Ylide Complexes without Auxiliary
Ligands
It was already found in 1972['601that the coordination of
monofunctional phosphorus ylides at nickel(o), for example by substitution of 1,5-~yclooctadiene,is accompanied
by a rearrangement of the ylide in the sense of a Stevens
rearrangement to give Nio complexes of the isomeric tertiary phosphanes.
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
Me2
IMe,P),PtCI,
A
tBu2MePCH2 f Bu2P
12lb
lPM
C,e Ylide
A
vpt'PMe3
919
Both with Ni" as well as with Pd" and Pt", the nitrido-,
methanido- and boronato-bis(phosphoni0methanide) anions X(PMezCH2h0yield isoelectronic, square planar complexes 127, the structures of which have been determined
by X-ray diffraction["*. 165-1691.
x
x
0:
:fe
talated ylide 129 with PMe,-complexed NiClz; the structure of 131 has been determined X-ray crystallographicallY[1711.
M=Ni. Pd, P t
122
X=N.CH.BH2.BMe2
129
9.2. Ylide Complexes of Alkylmetal Compounds and
Metal-Phosphinomethanides
The PMe, complex of (CH3)2Nireacts with Me3P=CHZ
with partial displacement of the tertiary phosphane to
yield only the trans-configurated ylide complex 123. Already in the case of CH,NiCI, however, the reaction proceeds via the isolable intermediate 124 with transylidation
to the binuclear ylide-bridged complex 125, which resembles compounds of type 120b[1701.
Me2NilPMe313
-
Me3P
123
tB
MelCllNi(PMe313
Bis(dipheny1phosphano)methanide ligands and simple
ylides form the similarly stable complexes 1321172,1731.
Xray data again reveal the environment of the metal to be
square planar[1721.
9.3. Ylide Complexes of
- kI C H 2
Me
Me3PCH2
131
Q
Me
Me3PCHz
130
-
r P M e 3
Ylide
Ni0
Cle
Me3P'
\--Me3
Me\
Carbonyl- and Cyclopentadienylnickel
It was first demonstrated 15 years
that phosphorus ylides can displace at least one CO ligand from
Ni(CO),. The tetrahedral Nio complexes that result have
since been synthesized in large numbers"02*'601
and one of
them has been studied crystallographically~17s1.
12L
NilCO)L
Reaction of the cod-complex of (CH3)?Pt with ylides
leads quantitatively to the cis-configurated complexes
1261'561.
MezPtIcodl
-
Me
\
+ZR3PCH2
-cod
r P R 3
126
Me
(CH,),PtI reacts with Ph3P=C=PPh3 in a complicated
reaction to form the Ptrl
complex 128, in which all the CH3
groups have been cleaved from the metal['211;127 is postulated as an intermediate.
-co
0
R'
I m
IC0)3Ni-C-PRg
I
133
R"
1351176,1771
C,H,Ni(PPh,IBr
C, HgNilPP hg 12fe
920
R3P=CR'R"
The CO ligands are not attacked by silylated ylides (in
spite of the higher affinity of silicon for oxygen)11021
and
even the carbodiphosphorane Ph3P=C=PPh3 reacts with
exclusive displacement
of CO to give
133
(R'R"= PPh3)[1001;
however just as little is known about the
structure of the product as about that of the complex
obtained
on reaction
of (Ph3P)2Pt(C2H5) and
PH3P=C=PPh3'981).
Bis(q5-cyclopentadienyl)nickel reacts with Ph3P=CH2
with exchange of one of the two C5H5 rings to give the
complex 134, which is accessible in an analogous manner
from C5H5Ni(PR3b@.
Appropriate stoichiometric reactions
of the C,H,Ni(PR,)X precursors lead to the mixed cations
IC, H512 Ni
The highly stable trans ylide complex 131 can be synthesized via the precursor 130 formed on reaction of the me-
+
Ph3P=CHX
HY
Ph,P=CHX
IC5H5NilCHX-PPhg1210ye
/-
13L
1
P hg P=C HX
Ere
LC5H~Ni(PPhglCHX-PPh310Bre
X=Me.Et,nPr.nBu.CI.OMe, "HX'':Me2;
135
Y=Br.PFC
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
9.4. Formation of Ylide Complexes via
9.5. Ylide Complexes of
Haloalkylmetal Precursors, from Olefin Complexes
or by CHz-Insertion
Palladium(I1) and Platinum(I1) Halides
It was first reported in 197@'781
that (R3P)4Pt0complexes
undergo oxidative addition with dihalomethanes, leading,
uia haloalkylmetal intermediates, to ylide comp l e ~ e s " ~ ~ ,Under suitable reaction conditions it is possible to isolate the ClCH,Pt intermediate ; hence, the second reaction step can be monitored separately"81*'821.
PtIPPh,),
CH2ClI
cis-I(Ph3P12Pt(CIlCH2PPh31e~
136
Complexes of Pd" and Pt" halides with phosphorus
ylides, in which the ligand is bound to the central atom by
a metal carbon bond, have been synthesized on a large
scale and subjected to detailed studies. This is particularly
true for the group of compounds with ylides which bear an
additional donor function and can, thus, function as bidentate ligands. PdX, and PtX2 react with phosphorus
ylides in a molecular ratio of 1 :2 to give products of the
type 144. The oldest examples are the adducts of PdCI2 or
PtCl, with Ph3P=CH-CO-Ph['861.
R'
PPh
(Ph3P)2Pt(CH2C11CI
3
l(Ph3Pl~Pt(CI)CH2PPh31"Cle 138
On changing the phosphane and in the presence of Lewis acid catalysts the compounds are inclined to isomerize
(cis/trans or ligand isomerization). A typical structure has
been elucidated X-ray crystallographically[''9~'801.
The six-fold coordinated platinacyclobutane compound
139 decomposes in the presence of tertiary phosphanes
with formation of a series of hydrocarbons (such as cyclopropane, propylene, ethylene, and methane), whose formation is best interpreted in terms of an equilibrium, the position of which is influenced by the PR3 components. It has
not yet been possible to demonstrate directly the presence
of the postulated ylide complex 141['831.
R,'
M=Pd,
M=Pd;
M=Pd,
COOEf
F't; X=CI; R=Ph; R'=COPh"861
X=C1; R=Ph; R=COPh, COMe, COOMe1'871
Pt; X=CI, SCN; R=Ph; R=COPh, COMe, COOMe,
COCN"881
The binuclear M" complexes 145 form the corresponding 1 :1 compounds with an auxiliary ligand L1lS9l:
x
1L5
\M/
x
\Mk
x
i
\:/
Ph3P-CHCOPh
e
/ \/
/ \ / \
L
X
X
X
PPh3
1L6
HC
I
/t=o
Ph
M=Pd; X=CI; L=PMe,, PMezPh
M = R ; X=CI, Br, I; L=PMe,, PMe2Ph
139
1LO
Reaction of a 1,5-cyclooctadiene-Pttl complex
(R2N4)Pt(cod), R = 4-C6H4N02,with PEt3 furnishes a o,n-
Reaction of bisbenzonitrilepalladium(r1) chloride with
MePh2P=CHSiMe3 yields first the binuclear ylide complex 147, which reacts further with diolefins and AgPF, to
MePh2P-CHSiMei
1 YiMe3 1
e
St Me3
cod
C12Pd-C H
PPhzMe
@
alkylmetal compound bearing a newly introduced onium
center in position 2 of the ring. The structure has been elucidated by X-ray diffraction
On treatment with diazomethane, the hydridodiplatinum
complex 142 yields a pmethylene-ylide complex 143
bearing a terminal Ph3PCH2 function. The mechanism of
formation is unknown, but the structure has been confirmed by X-ray
give a mononuclear cation complex 148. The product has
been identified by X-ray structure a n a l y ~ i s " ~ ~ . ' ~ ' ~ .
With Ph,P=CHCOR' (R'=Me, Ph), the 1 : 2 complex
CI,Pd(ylide), is formed['921.Complexes of the same ylide
type can be obtained on starting with a salt precursor 149,
which yields the binuclear complex 150 even on treatment
with the weak base sodium acetate. The binuclear complex
150 is cleaved to mononuclear species in donor solvents
such as dimethyl sulfoxide (DMS0)r'931:
IPh3?-CH2-COR'12Pd2Clge
1L9
1L2
Angew. Chem. Int. Ed. Engl. 22 (1983) 907-927
1L3 .R=Ph
AgPF6
2
NaOAc
[lPh3P=CHCOR')PdC1212
150
.sol
CI2Pd(Ylidel-DMS@
92 1
Ylides with a terminal phosphane function in the side
chain are excellent chelaters of metal dihalides. The formation and structure of the products 151 have been studied in great detail.
Triphenylphosphoniocyclopentadienide, which has already been used in 156 as a donor, is also represented in a
whole range of other Pd" and Pt" complexesfZo5~
'061.
10. Complexes of Phosphorus Ylides with
Copper, Silver, and Gold
R3
n=l; M=pd, Pt; X=C]; R1=R2=R3=Ph['94-1971; R 1 = R 2 = p h
R3= Me, OMe['951
n = 1; M = Pd, Pt; X = C1, Br, I, SCN; R' = R2= Ph, R3= Ph, Me,
OMe, OEt11981
n=O; M=Pd, Pt; X = C l ; R'=R2=Ph, R3=Ph, Me, OMe['98'
n = l ; M=Pd, Pt; X = C l ; R'=R2=CH2Ph, R3=Ph11991
Ionic complexes 152 are formed with the same ylides
from precursors of type 145; again L has been varied
within wide limits[2001.
The coinage metals form particularly simple complexes
with ylides, which are distinguished by their stability towards oxidation and hydrolysis. They also belong to the
most thermally stable organometallic compounds of these
'O81.
Homoleptic ylide complexes exist both with terminal
monodentate ligands and with bridging bidentate ligands.
The former are represented by the ionic compounds 158,
which are particularly common for M=Au with various
residues R[z09-z171.
R'
(L)MX
+
2R3P=CHR'
IR3P~-CH-Me-~H-PR~1Xe
I
158
R'
n=O, 1; M = P d , Pt; R'=RZ=R3=Ph[2011; L=PPh3, P(C6H,,)3r
AsPh3, PPh2Me, P(OMe)3, SbPh, py, pyMe, pyrazole
The 1 :1 complexes 153 of PdCl, and PtCI2 with a fluorene ylide probably also have a similar five-membered ring
structure as in 1511641.
Allylphosphonium salts react with NazPd2C16 or
Na2PdC1, in methanol in the presence of even weak bases
(acetate) to give phosphonioalkylide complexes 154. The
stepwise removal of halogens with silver salts of poorly
coordinating anions leads to binuclear dications 155 or, in
the presence of donor molecules, to mononuclear complexes 1561202-2041.
X = C I (Br); M=Cu,
M=Cu,
M=Au;
M=Au;
Au; R = M e ; R = H , SiMe3
Ag, Au; R = P h ; R = H ,Me, iPr
R=Et, ~ B u R'=H;
;
R=NMe2; R = H
[(Ph,P-CHCOPh)2Au]eAuChQ"861
[(Ph3P-C(CH2)2)2Au]"Cle
159
Many of these complexes dissolve in water without decomposition and are easily characterized analytically and
spectroscopically. The triphenylphosphoniocyclopropanide complex 15912181
is a special case, which exhibits quite
exceptional properties. In spite of the ring strain at the
ylide C atom, the species nevertheless exhibits high stability. The most important reaction of these complexes, with
their linear configuration and a coordination number of
two for the central atom, is transylidation with excess
ylides (as catalyst) to binuclear complexes 1601219-2281:
M=Cu, Ag, Au; R=Me, Et
M=Cu, Ag; R=Me, Ph
M = A u ; R2=(CH2)4,(CH2)5,(c-C3HS)Mer
(c-C5H9)Me
According to crystal structure data, these heterocycles,
which without exception are stable and colorless, contain
two almost parallel C-M-C axes of doubly coordinated
metal atoms which are 2.843
or 3.023 (Au)1225,2261
and theoretical calculations[''*~
apart. UV
indicate that the metal-metal interaction is very weak. The
eight-membered ring of type 160 can also be incorporated
in p o l y m e ~ s f ~if' ~a,w-difunctional
~,
ylides are used (161):
Several complexes of types 158 and 160 have been
tested for their pharmacological activity in polyarthritis.
The effect is comparable with that observed with conventional p r e p a r a t i o n ~ [ ~ ' ~ . ~ ~ ~ J .
A
R'=Ph, Et; R Z = H , Me; R 3 = H , Me; Y=BF,,
LL=cod, Ph,P=C(CH),
922
PF,, S03CF3;
Angew. Chem. Inr. Ed. Engl. 22 (1983) 907-927
[LAu-CRZ-PR31@X0 172, X = C l (Br);
The auracycles 160 easily undergo oxidative addition on
reaction with halogensIZ20*2211,
d i s ~ l f i d e s [and
~ ~ ~alkyl
'
halL = Me3P; R'=H, H/SiMe3, SiMe3; R"= Mec209,2'31
i d e ~ " ~ whereby
~],
bicyclic gold(rr) compounds 162 to 164
L = Ph,P; R = SiMe,; R ' =
L = Ph3P; R = H, H/Me, W/iPr; R" = Phcz161
are first formed in which a transannular Au-Au bond has
X=NO,;
been confirmed by X-ray diffraction[2211,by magnetic
L=Ph3P; R'=H/COPh; R"=Ph"s61
measurements and by M o s s b a ~ e rand
~ ~Au-ESCA
~ ~ ~ ~ ~ ~ ~
studies122Z1.
173, M=Cu, Ag; R = P h ; R'=H, Me, iPr[2361
CI&CH-?R:,
I
R'
PPh3
€34
CIM-C
174, M-Cu, Ag, Au"~"
I@
\\
PPh,
~ A u - PR i
R2Pm
,.\
e
L&-p/
m; CH
175, R=Me, Et, tBu; R=PhI'73.23s1
K2
\
CHZXZ
/x
x2
165
R2PT
7PR2
\AuJ
x'
166
Further oxidation of 162 leads, with cleavage of the
Au-Au bond, to the binuclear Au"' complex 165[220,2221.
In addition, some square planar complexes of trivalent
The CH31 adduct 164, whose structure was originally degold with various alkyl-, halogen-, and ylide-ligand combiduced solely from NMR
has recently been unenations have been d e s ~ r i b e d [ ~ ~.~0,ne
~ 'example,
~~~~~-~~~~
quivocally confirmed by X-ray structural analysis[2321.
On
177, has also been investigated by X-ray diffraction and by
further alkylation it yields the unstable, unsymmetrical
ESCA spectroscopy. The results show that the ylides are
dialkyl derivative 167, which easily undergoes thermal depowerful donor ligands that have practically no backcomposition with elimination of ethane to give 160.
bonding acceptor effect[2381.
Addition of dihalomethanes to 160 results in formation
Three isoelectronic auracycles 182 are derived from
(CH&Au@ and have been studied in detail, particularly by
of a CH, bridge between the metal atoms, raising their
X-ray diffraction. The folding of the six-membered ring
coordination number from 2 (in 160) to 4 (in 166). Crystal
structure analysis reveals a square planar ligand arrangeinto the chair conformation depends on the nature of the
ment as is to be expected for A u " ' ( ~ ~ ) [A~gold(1)
~ ~ ~ . combridge element X; the environment of the gold atom, howplex of the double anion 62 probably has the twelve-memever, remains practically ~ o n s t a n t [ ~ ~ ~ ~ ~ ~ ~ l .
bered ring structure l6flrZ3".
Me,AuCtipMe3
- 2' TMe3
+HCI
-CHk
168
A large variety of ylide complexes of the coinage metals
with additional ligands are also known and exist as cations
or neutral molecules. Formulas 169 to 176 summarize the
individual examples with monovalent central atoms :
177
Me-Au-C'Hz
I
Me
PMe3
PMe3
1
I
Me-Au-CH2
@PMe3
/
CP
Me
178
1
Me3PCHZ
lX~AuiCH2PMe31~l%ls
181
Me3SiCHzCuCHzPMeZ 169J211J
[Me3PCHCuCHPMe3JeCle 170, R = H , SiMeJZ1l1
I
I
R'
RAuCHPMe, 171, R=Me, CHZSiMe3; R = H , SiMe,
I
R'
R'
Angew. Chem. Jnr. Ed. Engl. 22 fJ983)907-927
923
11. Complexes of Phosphorus Ylides with the
d'O-Metals Zinc, Cadmium, and Mercury
ment. The trend in interest is unmistakably towards the intentional or accidental formation of ylides in situ at the
metal atom, rather than the planned introduction of "preconstructed" ylides into a complex. The latter development seems temporarily complete now that the most important modes of formation and interactive mechanisms
are known. However, these constitute but the preliminary
studies necessary for an understanding of some of the
complicated reactions in the system metal/phosphane/C,
building block.
The guiding principle in the chemistry of the ylide complexes of the elements of the second subgroup is-similarly to that for the whole organometallic chemistry of
these elements-strongly directional o-bonding with smaIl
coordination numbers.
Homoleptic coordination compounds with monodentate
ylide ligands are the cationic complexes 183, which have
been synthesized in large numbers and with a wide range
of ~ u b ~ t i t u e n t ~ [ " ~ - ~ ~ ~ .
R'
I
R'
I
183 IR~P-CH-M-CH-PR~lZ@
2Xe
2R3P-CHR'
R~P-CHR'
7
M = H g ; R = P h ; R'=HLZ"', COOMe, COMe, COPh[24'J
M = H g ; R = M e ; R'=H, SiMe312431
MX2
R'
M = Z n , Cd, Hg; R = P h ; R'= Me, iP$2441
I
IX-M-CH+R31eXe
18L
M=Hg; X=CI; R = M e ; R'=H[z43]
M = H g ; X=CI; R = P h ; R'=COPh[z42'
M = H g ; X = I ; R = M e ; R'=SiMe3[Z451
If only small amounts of ylide are available the 1 :1
complexes 184 are formed, alkyl metal analogues of which
are also known [R"-Hg-CHR'-PR,]X
(R"= Me12431).
The high donating power of the double ylides is demonstrated in the unusual chelate complex 185, which is one
of the few tetracoordinated organomercury compounds
known12461.
Me?
Open chain and cyclic double ylides also yield complexes with the two lighter metals of the second subgroup,
i.e. zinc and ~ a d m i ~ m. As
I ~i.s~to~be- ~expected
~ ~ ~ such
complexes can be ascribed formulas of the type 63,76 and
10912481.
In passing, the cadmium complexes of fluorinated
phosphinatomethylide anions are worth mentioning. .Their
relationship to real ylide compounds is only of a formal
nat~re[~~'1.
Finally, however, the mercury complexes of triphenylphosph~niocyclopentadienide[~~~~~~~~
are of greater interest. In the organometal compounds formed on reaction of
this ylide with mercury(r1) salts, the metal atom is ql-coordinated to the C-3 atom of the C5H4ring, as has been confirmed by an X-ray structural analysis of the adduct
[Ph3P-C,H4- HgI,I2. This unusual type of structure has
otherwise never been observed amongst ylide complexes.
12. Outlook
The present review represents a snapshot glimpse at a
field of chemistry that is in the process of rapid develop924
Work in the author's laboratories was perfonned by a
large number of co-workers, whose skill and enthusiasm are
grateful@ acknowledged. n a n k s are also due to the Fonds
der Chemischen Industrie, Deutsche Forschungsgemeinschaj2, Hoechst AG, Degussa AG, and Proctor 8 Gamble
Ltd. for generous and continued support.
Received: July 25, 1983 [A 476 IE]
German version: Angew. Chem. 95 (1983) 980
Translated by Dr. F. Hampson, Saarbriicken
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Electron-Rich Half-Sandwich ComplexesMetal Bases par excellence
By Helmut Werner’
Dedicated to Professor Ernst Otto Fischer on the occasion of his 65th birthday
Electron-rich, half-sandwich complexes of the type C,R,ML2 or C,R,MLL‘ are built up of
an aromatic five- or six-membered ring, a d8-metal, and either a pair of two-electron donors
or an equivalent chelating ligand. Such complexes behave like Lewis bases and react with a
wide variety of electrophiles, El or ElX, to form products with a new metal-element bond.
According to their reactivity they are comparable to the Vaska-type compounds. Certain of
the products obtained after addition of the electrophile undergo interesting subsequent
reactions in which, for example, metal complexes containing molecules that are unstable in
the free state, such as CS, CSe, CH$, CH,Se, CH2Te, CH3CHS, CH3CHSe, CH2=C=S,
CH2=C=Se, and CH2=C=Te are formed. Moreover, cycloadditions as well as reactions
with coordinatively unsaturated transition-metal compounds which result in formation of
heterometal binuclear complexes demonstrate that the metal bases C,RnMLz and
C,R,MLL’ are valuable synthetic building blocks. Furthermore, very recent investigations
have indicated links between metal basicity and the problem of C-H activation.
1. Introduction
Half-sandwich complexes are almost as old as ferrocene-the progenitor of all sandwich compounds. Shortly
after the structure of dicyclopentadienyliron had been accurately determined and analogous metallocenes had
been synthesized, Fischer und Hafner”’ reported the
synthesis of tetracarbonyl(cyclopentadienyl)vanadium,
C5H5V(COk. Later work-involving highly competitive research efforts in Munich and Harvard-produced the homologous manganese and cobalt complexes C5H5Mn(C0)3
and C5HSCo(C0)~2*31.
Because the metal atom in these
compounds is enclosed on only one side by a C,H,-disk
(as in the metallocenes) but is bonded to conventional ligands on the other side, the structure resembles a “halfsandwich”. This term was later applied to similar compounds: instead of CO, ligands such as NO, halogenides,
isocyanides, phosphanes, and olefins were now coordinated to the metal, and instead of CSHs other ring systems
such as C4H4,
C7H7 or CsH8 were employedl4].
[‘I Prof. Dr. H. Werner
Institut fur Anorganische Chemie der Universitxt
Am Hubland, D-8700 Wiirzburg (Germany)
Angew. Chem. Int. Ed. Engl. 22 (1983) 927-949
There was no indication in early publication^[^.^^ that
half-sandwich complexes can behave as metal bases, i. e.
that they react with electrophiles, El or ElX, to form new
metal-element bondscs1.Such behavior was hardly to be expected since both the carbonyl(cyclopentadieny1)metals
mentioned above and most of the analogous compounds
with the general formula C,H,ML,
are 18-electron complexes. At least in a formal sense these possess no lone pair
of electrons capable of forming a covalent bond with a
Lewis acid. Vaska-type compounds trans-[MX(CO)L,]
(M=Rh, Ir; X=C1, Br, I, N,; L=PR3, P(OR),, AsR3 etc.)
do possess such a lone pair of electrons and react not only
with known electrophiles, e. g. HCl, CH31, CH,COCl, BX3,
SOz, etc. but also with H2 or R3SiH by oxidative addition@’. The Vaska-type compounds are thus typical metal
bases, and it is not only thanks to their chemistry that the
concept of metal basicity has become increasingly known
since the beginning of the seven tie^'^^.
The central atom in the trans-[MX(CO)L] complexes is
a d8-system, as is the case in the dicarbonyl(cyc1opentadieny1)metal compounds C5H5M(C0)2(M = Co, Rh, Ir). In
an article which indicated the great significance of the nucleophilicity of transition-metal atoms in complexes having full valence shells, Wilkinson et
mentioned that
0 Verlag Chemie GmbH, 6940 Weinheim, 1983
0570-0833/83/1212-0927 $ 02.50/0
927
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