Metal-carbonyl dimer

Metal carbonyl dimers have proved to be useful reagents for the synthesis of mixed-metal clusters. This is particularly true for Fe and Co clusters since Fe2(CO) and Co2(CO)8 are readily available starting materials. The prediction of the reaction products is usually fruitless. However, examination of the available data indicates that the initial dimeric unit is preserved in approximately half of the reactions. Typical examples are illustrated in Eqs. (4-6) (102), (7) (28), and (8-10) (121). 

Aside from the carbonyl group, the Cp ligand and its analogs are probably the most common ligand systems in organometaUic chemistry. This is also reflected in the large number of cyclopentadienyl metal carbonyl dimers and their derivatives that have been studied. For molybdenum, direct reaction of molybdenum hexacarbonyl and dicyclopentadiene is one of several routes to [CpMo(CO)3]2 (equation 14). 

Substitution reactions on metal-carbonyl dimers (see Dinuclear Organometallic Cluster Complexes) and clusters see Polynuclear Organometallic Cluster Complexes) occur by replacement of a CO with an entering ligand ... 

Metal-carbonyl Dimers. The reactions that have been most often studied are those of the group 7 dimers, Mn2(CO)io, Tc2(CO)io, Re2(CO)io, and MnRe(CO)io. Kinetic studies on these dimers show the rate law ... 

AsPh3) occurs by a primarily hgand-independent mechanism, probably CO dissociation. For more nucleophilic entering ligands (PBu3, CN-t-Bu), the hgand-dependent path still predominates. The CO dissociation rates from Ir4(CO)uL show the same trends with changes in L as mononuclear complexes and metal-carbonyl dimers. 

Reaction of [Ti(CO)6] with MeOH and PhOH gives the metal-carbonyl dimers [Ti(CO)4(/x-OR)]2 (R = Me, Ph). The structure and reactivity of these complexes suggest that the bridging alkoxy ligands function as r-donors satisfying... 

Reactions with organometalllc substrates. One useful reaction of this and other trialkylborohydrides is cleavage of metal carbonyl dimers to metal carbonyl anions (equation I). The by-products are Hj and B(C2Hs)3. Another useful reaction is the generation of anionic formyl complexes (equation II). Sulfur (Sg) can be cleaved to give either LijS or Li2S2 (equations III and IV). Disulfides are cleaved by this reaction to lithium mercaptides. 

The most general route to anionic or hydridic dimeric complexes utilizes a metal carbonyl dimer as a starting complex. Reaction of Mn2(CO)io with NaBH4 and subsequent treatment with phosphoric acid led to H2Mn2(CO)9F... 

Metal carbonyl dimers are prepared with biscyclopentadienyl linked ligands. The following sequence of reactions is utilized to prepare cobalt dimers E... 

This reaction is useful for preparation of metal carbonyl dimers, such as MnRe(CO)io, MnRe(CO)9L (both isomers), and Cp(OC)3WMn(CO)s ... 

The reaction of metal carbonyl dimers with silicon hydrides also probably involves an initial oxidative addition step. Chiral silyl-cobalt and silyl-manganese carbonyl complexes have been obtained through the reaction of optically active organosilicon hydrides with metal carbonyls65 68 (equation 15 and 16). Phosphine-substituted cobalt complexes were similarly obtained by reaction of a chiral hydrosilane with Co2(CO)6L2 [L = PPh3, P(OPh)3, P(c-C6Hn)3]69. 

The kinetics of the oxidations of trans-[Tc(dppe)X2] (X = Cl , Br ) by Co(III) amine complexes have been investigated in pure and mixed DMF and CH3CN solvent systems, with an observed increase in the rate constant with DMF concentrations. The rate constants for the reductions of three metal carbonyl dimers [M2,M = Mn(CO)5, CpMo(CO)3, and Co(CO)4] by [Re(CO)5r in THF (measured using infrared stopped-flow spectroscopy) do not parallel the reduction... 

The ability of PE spectroscopy to interrogate the whole range of valence orbitals of a molecular system has proved invaluable in investigating the electronic structure of metal-metal multiple bonds and small metal clusters. In the organometallic field, most attention has been focused on metal carbonyl dimers and small clusters, and their derivatives, on account of the high stability and volatility of these species. Metal chalcogen clusters of stoichiometry M4CP4E4 (E = O, S, Se) have also proved viable and fruitful candidates for PE spectroscopy. ... 

CYCLOPENTADIENYL METAL CARBONYL DIMERS OF MOLYBDENUM AND TUNGSTEN... 

Cyclopentadienyl Metal Carbonyl Dimers of Molybdenum and Tungsten ISl... 

All of these examples have been cases in which all of the valence d electrons on each metal atom have paired in either bonding or antibonding orbitals. For zero-valent metal carbonyl dimers only one of the many valence d electrons on each metal is shared hence, only single bonds are known for these systems. The reason why the earlier transition metals in nonzero valent oxidation states form up to quadruple bonds and the later zero-valent transition metal carbonyls form only single bonds can be understood by considering the electronic needs of these different metal centers. 

Turning to carbonyls containing more than one metal atom, Johnson has proposed a new mechanism for the substitution reactions of metal-carbonyl dimers Corraine and Atwood have produced a number of papers dealing with carbonyl anions in reaction with carbonyl dimers. The thermodynamic and kinetic factors that control the reactions are also discussed 2. Heaton et al have discussed the electron-microscopy of transition-metal carbonyl clusters 25,26 Butler et al have published a comparison of photoacousdc, attenuated total reflection, and transmission infrared-spectra of crystalline organoiron(ll) carbonyl-complexes 27. 

It is easy to determine the bond order of metal-carbonyl dimers, because they follow the 18-electron rule. This remains true if the dimer contains both Cp and carbonyl ligands. For instance, the dimer [Cp(CO)2Mo-Mo(CO)2Cp] can be written under the form [Mo(CO)2Cp(X)y] if y is the Mo-Mo bond order (each Mo being a Xy ligand for the Mo neighbor), i.e., MoL4Xy+i. 

Brown has shown that metal-carbonyl-hydride complexes such as [Re(H)(CO)s] can nndergo CO substitntion by a phosphine (PBus) according to the H-atom-transfer-chain mechanism A classic type of initiation to introduce the radical species into the chain is to photolyze the metal-carbonyl dimer, which generates the reactive 17-electron metal-carbonyl monomer ... 

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