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Highly-Reduced Metal Carbonyls.

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HIGHLIGHTS
Highly-Reduced Metal Carbonyls
By Wolfgang Beck*
Anionic carbonyl complexes (carbonylmetalates) contain
metals with formal negative oxidation numbers and constitute one of the most important classes of compounds in
organometallic chemistry. Since the first systematic studies
of carbonylmetalates by Hieber et al.['] these compounds
have proven to be valuable starting compounds for the synthesis of a variety of-even multinuclear-metal carbonyls
and their derivatives.['] They are also used in organic synthesis.f31
The first anionic carbonyl complexes [HFe(CO),]O and
Fe(C0):O were obtained by reaction of Fe(CO), with alkaline hydroxide solutions 14] (Hieber base reaction).
Fe(CO),
+ 3 OHe
-
[HFe(C0),le
+ CO:e + H,O
Kruck et
interpreted the mechanism of this reaction
in terms of an attack by hydroxide at the C atom of a CO
ligand with elimination of carbon dioxide.
The simplest and most straightforward entry to carbonylmetalates is via the reduction of metal carbonyls, usually
with alkali metals, whereby a CO-ligand (two-electron donor) is formally replaced by two electrons or a metal-metal
bond is reductively cleaved.
Fe(CO),
[(OC),Re-Re(CO),]
+ 2 Na-
+ 2 Na-2
Na,[Fe(CO),]
Pioneering work by Behrensf6I in 1941 already showed
that this reduction is effectively carried out in liquid ammonia. Using the 18e rule a series of mononuclear neutral and
anionic carbonyl complexes results in which the central atom
in each case has a noble gas electron configuration (Table 1).
Table 1. Mononuclear neutral and anionic carbonyl complexes with noble gas
electron configuration [a].
tetrahedral
Ni
Co,Rh,Ir M(CO):e
Fe,Ru,Os
Mn,Re
Cr,Mo,W (M(CO)!e
V,Nb,Ta
Ti,Zr,Hf
Ni(CO),
M(C0)p
M(CO):e
M(CO):e
M(CO):e
(M(C0):O)
(M(CO):e)
Na[M(CO),]
+ 2 Na- Na,[M(CO),] + CO
+ 4 Na Na,[M(CO),]
+ 2 Na-Na,[M(CO),]
+ CO
Na[Co(CO),]
+ 2 Na-
Na[M(CO),]
[M(CO),(tmeda)]
-
Na,[Co(CO),]
(M = V,Nb,Ta)
(M = Cr,Mo,W)
(M = Mn,Re)
+ CO
The synthesis of hexacarbonyltitanate, -zirconate, and
-hafnate( - II), the first stable binary carbonyls of elements
of group 4,was achieved, inter aha, by reductive carbonyiation of [M(CO),L] with potassium naphthalenide as reducing agent in the presence of cryptands!"]
+ CO
Na[Re(CO),]
Metal
A few years ago one would scarcely have thought of the
existence of anions such as Cr(C0):" !
Synthesis: Highly-reduced metal carbonyls and their
derivatives were prepared by Ellis et al. with alkali metals or
alkali metal naphthalenides as reducing agents in liquid ammonia, in hexamethylphosphoric triamide, or even in THE
Singly charged anions or metal carbonyls with donor ligands
(amines, phosphanes) were employed as starting compounds
(see, e.g., Ref. [8,9]).
Structure
trigonalbipyramidal
M(CO),
M(CO)?
M(C0):O
M(C0):'
(M(C0)te)
octahedral
M(CO).?
M(CO),
M(C0)F
M(C0):O
The titanium and zirconium compounds are also accessible under similar conditions directly from the metal tetrachlorides. The "exhaustively" reduced carbonylmetalates
M(C0):'
(M = V,Nb,Ta), M(C0):'
(M = Ti,Zr,Hf) and
M(C0):' (M = Cr,Mo,W) have not yet been characterized;
Ellis et al., however, have been able to isolate their corresponding triphenylstannyl derivatives (see below).
Properties and reactivity: The solvent-free salts of the triand tetraanionic carbonylmetalates are, as expected, strong
reducing agents; they are extremely pyrophoric. The isolated
salts Na,[M(CO),] (M = Cr,Mo,W) or Cs,[Ta(CO),] can
explode under argon (!), even without external interference.
The extremely sensitive compounds were therefore mostly
employed in situ.
Carbonylmetalates, e.g. Cr(C0):' can reduce CO, .I1 'I
Cr(CO)Ze
+ 2 co,
-
Cr(CO),
+ CO:e
[a] Derivatives of the compounds quoted in parentheses are known
The compounds within a column are iso-structural. The anions in the same row are each derived from the one on the
right by replacement of a CO-ligand by two electrons.
Recently, Ellis et al."] were able to prepare new, highly
reduced, i.e. three- and four-fold negatively charged metal
carbonyls ("Ellis compounds") by a series of impressive experimentally demanding and systematically planned studies.
[*I
Prof. Dr. W. Beck
Institut fur Anorganische Chemie der Universitat
Meiserstrasse 1, W-8000 Miinchen 2 (FRG)
168
0 VCH Verlagsgesellschafi mbH. W-6940 Weinheim, 1991
Such a reaction is possibly responsible for the unexpected
formation of the novel p,-carbonato-bridged complex A,
which is formed along with the hydride [(OC),ReIr(H)(CO),Re(CO),] from Ir(CO):@ and [(C,H,)Re(CO),]@ on
exposing the reaction solution to
The protonation of carbonylmetalates is one of their most
important reactions and usually leads to hydrido complexes.
0570-0833/91/0202-0168$3.50+.25/0
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 2
Fe(C0):'
Re(C0):"
+ 2He-[H,Fe(CO),]
+ 2He-[H,Re(CO),]"
V(CO):o reacts with NH,CI in liquid ammonia with H,elimination (He as oxidizing agent) and formation of
[V(CO),NH,]@; the ammonia in this anion can be readily
replaced by stronger donor ligands L (isocyanide, phosphane).
V(C0);" + 2 He
+L
3
[V(CO),NHJe -NH; [V(CO),Lle
-H2
Carbonylmetalates react as nucleophiles with alkyl and
aryl halides, affording access to the important class of
organometallic complexes containing a metalkarbon 6bond.
Corresponding reactions can also be carried out with the
highly reduced Re(CO):@ anion (H,COTos = methyl p toIuenesuIfonate:[*]
The multinuclear complexes have interesting structures
containing transition metals with high coordination numbers. The synthesis, structure, and dynamic behavior of
(Ph,Sn),M(CO):@ (M = Zr, Hf) in solution is described in
this issue of "Angewandte Chemie".['I
Investigations on highly reduced metal carbonyls also led
Ellis et al. to novel carbonyl compounds of the early transition
Thus, the CO-richest derivative of the hitherto unknown Ti(CO), could be synthesized with the structurally characterized [Ti(CO),(Me,PCH,CH,PMe,)].
Finally, the bonding situation in these unusual carbonylmetalates should be briefly discussed. The high negative
charge leads to strong metalLC0 n-backbonding ; the negative charge is distributed over the antibonding n*-CO orbitals corresponding to M = C =0 ++ OM = C-O@.
With increasing negative charge the C - 0 bond is weakened, as shown by the wave numbers of the CO vibrations
(Table 2).
Table 2. CO vibration frequencies of binary metal carbonyls.
Metal carbonyl
P (CO) [cm-'1
Metal carbonyl
~~~~~
As we have already been able to show, protonation of
[(H,C),Re(CO),]@ with tetrafluoroboric acid affords the
doubly coordinatively unsaturated Re(C0):
a useful
organometallic Lewis acid I'3b1 for reactions with nucleophiles.
[(H,C),Re(CO),]"
+ 2 He
--t
Re(C0)F
Mn(CO),O
cr(co),
V(CO),e
Ti(C0);O
2090
1980
1860
1750
P (CO) [cm-'1
~
Ni(CO),
CO(C0)P
Fe(C0):O
Mn(CO)ie
Cr(C0):O
2050
1890
1790
1670
1460
The position of the CO bands of tetracarbonylchromate(- IV) corresponds almost to that of a C-0 single bond, i.e.
+ 2 CH,
In our studies we have used carbonylmetalates as nucleophiles for additions to coordinated unsaturated hydrocarbons. The addition of carbonylmetalates to unsaturated hydrocarbons in cationic complexes has proven to be a direct
and rational method for the synthesis of numerous hydrocarbon-bridged complexes.[' ],
the CO group can be formulated as a carbyne ligand.
Summarizing, it can be stated that these remarkable compounds with metal atoms in the lowest known oxidation
states have enriched the chemistry of metal carbonyls considerably." Further interesting results can be expected.
German version: Angew. Chem. 103 (1991) 173
[I] W. Hieber, W. Beck, G. Braun, Angew. Cheni. 72 (1960) 795; Angew. Chem.
Int. Ed. Engl. 1 (1961) 65. W. Hieber, Adv. Organomet. Chem. 8 (1970) 1.
[2] J. E. Ellis, J. Orgunomet. Chem. 86 (1975) 1.
[3] Cf., e.g., Collman's reagent Na,Fe(CO), for the synthesis of aldehydes and
ketones. [HFe(C0),le finds many uses in organic syntheses and catalysis
(J.-J. Brunet, Chem. Rev. 90 (1990) 1041).
[4] W. Hieber, F. Leutert, Nutnrwissenschuftien 19 (1931) 360; 2. Anorg. ANg.
Highly reduced metal carbonyls are especially useful for
the synthesis of multinuclear complexes with metal-metal
bonds between main-group and subgroup elements, which,
e.g., are formed by reaction with chlorotriphenylstannane or
chlorotriphenylphosphinegold.
According to the studies of Ellis et al. triphenylstannyl
derivatives of almost all highly reduced metal carbonyls are
known.
[Co(CO), (SnPh,),le( s Co(CO):"),
(OC),Zr(SnPh,
),IzD(
2 Zr(CO):"),
[W(CO),(SnPh,),]z"(
[Ti(CO),(SnPh,),]2e
Angen. Chem. hi.Ed. Engl. 30 (1991) No. 2
0 VCH
sW(CO);'),
( G Ti(CO):e).
Chem. 204 (1932) 145.
[5] T. Kruck. M. Hofler, M. Noack, Chem. Ber. 99 (1966) 1153.
[6] H. Behrens, Adv. Organornet. Chem. 18 (1980) 1.
[7] Part 29 of Highly Reduced Organometallic Compounds: J. E. Ellis, K.-M.
Chi, A.-J. DiMaio, S . R. Frerichs, J. R. Stenzel, A. L. Rheingold, B. S .
Haggerty, Angew. Chem. 103 (1991) 196; Angew. Chem. I n t . Ed. Engl. 30
(1991) 194 and earlier communications.
[8] G. F. P. Warnock. L. C. Moodie, J. E. Ellis, .IAm. Chem. So<. 111 (1989)
2131.
[9] J. E. Ellis, P. T. Barger, M. L. Winzenburg, G. F. Warnock. 1 Orgunornet.
Chem. 383 (1990) 521.
[lo] J. E. Ellis, K.-M. Chi, J. Am. Chem. Soc. 112 (1990) 6022.
[ l l ] G. R. Lee, J. M. Maher, N. J. Cooper. J. Am. Chem. Soc. 109 (1987)2956.
1121 J. Breimair. C. Robl, W. Beck, .I Organomet. Chem., in press.
[13] a) E. Lippmann, W. Beck, unpublished; b) W Beck. K. Siinkel. Chem. Rev.
88 (1988) 1405.
[14] W. Beck, B. Niemer, J. Breimair, J. Heidrich, J. Orgunornet. Chew!. 372
(1989) 79; W. Beck, B. Niemer, B. Wagner, Angen. Chem. 101 (1989) 1699:
Angew. Chem. Int. Ed. Engl. 28 (1989) 1705.
[15] J. E. Ellis, Polyhedron 8 (1989) 161I.
[16) Note udded in proof? A detailed review on "Highly Reduced Metal Carbony1 Anions" has been published (J. E. Ellis, Adv. Organomet. Chem. 31
(1990) 1.
Verlagsgesellschaft mbH. W-6940 Weinheim. 1991
0570-0833/91/0202-0169 $3.50+ ,2510
169
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