The Carbonyl Ligand

Both a donation (which donates electron density from a bonding orbital on CO) and n acceptance (which places electron density in CO antibonding orbitals) would be expected to weaken the C-0 bond and to decrease the energy necessary to stretch that bond. 

The charge on a carbonyl complex is also reflected in its infrared spectrum. Three isoelectronic hexacarbonyls have the following C-0 stretching bands 12 

Positions of infrared absorptions may vary slightly depending on solvent, counterions, or other factors. 

Exercise 4-2 On the basis of the carbonyl complexes shown in the preceding table, 

Let s examine one more series of isoelectronic transition metal carbonyl complexes  

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Imidazole with [Re(CO)3(phen)Cl] or [Re(CO)3(phen)(CF3S03)] in the presence of sulfuric acid gives [Re(CO)3(phen)(im)]2SO (95ICA(240)169). Imidazole with [Mri2(CO) g] gives [Mn2(CO)g(imH)] (84P707). This path involves the nucleophilic substitution of the carbonyl ligand. However, it is complicated by some redox... 

Bis(l-methylimidazol-2-yl)methane and -ketone with the dimer [Rh(CO)2Cl]2 in the presence of sodium tetraphenylborate give the dicarbonyl complexes 68 (X CHj, CO L = CO) where the carbonyl ligands may easily be substituted by the triphenyl phosphine ligands to yield 68 (X = CH, CO L = PPh ) (99JOM(588)69). The bis(l-methylbenzimidazol-2-yl)methane analogs of 68 (X=CH2 L=C0, PPhj) can be prepared similarly. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

These experiments confirmed the photolability of the carbonyl ligands of 1 and suggested that the initial photoproduct may be the coordinatively unsaturated species [Cp2Ti(CO)]. This compound recombines with CO at a much faster rate than decomposing, thus appearing to show no net reaction upon photolysis. 

While essentially all the metal carbonyl complexes for group 4B contain terminal CO ligands, only recently have some bonafide doubly bridging carbonyl complexes been reported. However, these complexes are hetero-nuclear, since the carbonyl ligand bridges a zirconium atom with the metal center of a late transition metal. 

Formally, in each of these cases the disproportionation produces a positive metal ion and a metal ion in a negative oxidation state. The carbonyl ligands will be bound to the softer metal species, the anion the nitrogen donor ligands (hard Lewis bases) will be bound to the harder metal species, the cation. These disproportionation reactions are quite useful in the preparation of a variety of carbonylate complexes. For example, the [Ni2(CO)6]2 ion can be prepared by the reaction... 

Figure 3. Structures A and B of the (H)2Co(CHO)(CO)s complex. The numbers are optimized bond lengths (in A) at stage one. The carbonyl ligands were held fixed at 1.09A, the optimized value from the stage one calculations on HCo(CO)k.

Although the steric environment around each terminus of the allyl unit is virtually identical, Faller showed that addition of a soft nucleophile occurs exclusively trans to the carbonyl ligand. It was clear that electronic rather than steric factors were responsible for this remarkable selectivity, and, even though the precise origin of the effect was not immediately understood, the principle of electronic differentiation had been experimentally demonstrated.1221... 

The formation of the saturated monometal fragments will depend upon the ratio of CO M in the cluster and also, to a lesser extent, upon the arrangement of the carbonyl ligands about the central Mm cluster unit. Clearly, the higher the CO M ratio, the greater the probability of ejecting the appropriate M(CO) saturated unit. Consider the osmium series Osm (m = 1-8) ... 

It did not prove possible to synthesize a substituent-free Ga complex with formula Cp (CO)2Fe Fe(CO)4 Ga (Scheme 13).43 Addition of bipy to 30 resulted in halide elimation, but the main group element in the product 31 was coordinated by the bipy ligand. Upon addition of dppe, however, substitution of the carbonyl ligands occurred instead along with halide ion elimination to produce the substituent-free Ga complex 32. It has a linear coordination environment (Fe-Ga-Fe angle = 176.01(4)°), and the Ga-Fe bond distances are much shorter than in those related adducts where donor ligands are also bound to the Ga atom.43 The authors attributed the non-observation of the carbonyl derivative to a need for an electron-rich metal center to stabilize the Fe-Ga bond via 7r-backdonation. 

Weiss studied68a the reactivity of both new complexes, and found that a variety of phosphines and phosphites would also convert the vinylcarbene complex 139 into the corresponding vinylketene complex (140), capturing one of the carbonyl ligands from the coordination sphere of the metal to become the ketene carbonyl. Only in the case of triphenylphosphine was the dicarbonyl(phosphine)vinylcarbene complex (141) isolated, which then required addition of carbon monoxide to convert it to the dicarbonyl(triphe-nylphosphine)vinylketene complex 140.a. This interconversion was reversible and proceeded quantitatively. 

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