Bonding carbon monoxide

Further examples of coordinate bonds are found in metal carbonyl complexes. Metal carbon (carbon monoxide) bond distances in a selection of (first-row) transition-metal carbonyls and transition-metal organometallics are examined in Table 5-11. As expected, Hartree-Fock models do not perform well. The 6-3IG model is clearly superior to the STO-3G and 3-2IG models (both of which lead to completely unreasonable geometries for several compounds), but still exhibits unacceptable errors. For example, the model shows markedly different lengths for the axial and equatorial bonds in iron pentacarbonyl, in contrast to experiment where they are nearly the same. Hartree-Fock models cannot be recommended. 

The PM3 semi-empirical model turns in a surprisingly good account of metal-carbon (carbon monoxide) bond distances in these compounds. While PM3 is not as good as the best of the (density functional) models, individual bond lengths are typically within a few hundredths of an A from their respective experimental values, and larger deviations are uncommon. In view of cost considerations, PM3 certainly has a role in transition-metal structural chemistry. 

As with metal-carbon monoxide bonds, the MP2/6-3IG model does not lead to results of the same calibre as those from density functional models (except local density models). The model actually shows the opposite behavior as 6-3IG, in that bond lengths are consistently shorter than experimental values, sometimes significantly so. In view of its poor performance and the considerable cost of MP2 models (relative to density functional models), there seems little reason to employ them for structural investigations on organometallics. 

In 1971 only two complexes of palladium(I) had been identified.65 Although the area has grown significantly, the relative paucity of palladium cluster compounds can be attributed, in part, to the surprising weakness of palladium-carbon monoxide bonds and in particular those where CO is bound terminally. In this chapter the chemistry of palladium(I) and clusters of palladium in other oxidation states will be considered. However, complexes containing organic ligands such as allyl and cyclopentadienyl will not be dealt with as this area has been reviewed recently in a companion volume.66... 

In the preceding Lewis formula of CO, each dash represents a pair of bonding electrons, and each pair of dots represents an unshared pair of electrons.) Compounds with carbon monoxide bonded to metals, some of which are quite volatile and toxic, are called carbonyls. 

Unfortunately the situation concerning metal-to-carbon monoxide bond energies is worse. The first obvious complication is the occurrence of different types of bonding of the carbon monoxide (terminal, edge, and face bridging), although there is experimental evidence that, as a first approxi-... 

Sung SS, Hoffmann R (1985) How carbon monoxide bonds to metal surfaces. J Am Chem Soc 107 578... 

The reduction of nitro aromatic compounds to isocyanates by CO is preferably carried out with Pd, in the presence of a Lewis acid as promoter. . Recent observations that carbon monoxide bonded to Pd2(CO)2Cl4 has a very high Vco (slightly above 2160 cm with a small dependence on the solvent) indicate the degree of ir-back donation from the metal to the carbonyl ligand is small, if any. 

The key variables influencing the color of semimoist pet foods are (1) the extent of saturation of carbon-monoxide-bonding sites, (2) the oxidation of iron in the meat protein, (3) the final concentration of meat or blood protein, and (4) the type of heat (dry or moist) and its process, which maintains the color of the carbon-monoxide-treated materials. The carbon monoxide treatment is indeed necessary to stabilize the color of the products (Hood, 1977). 

Carbon monoxide bonds to hemoglobin in the blood stream 200 times as efficiently as does oxygen. This ability to exclude oxygen from the blood is responsible for the toxic effects caused by carbon monoxide in the body. 

These differences in reactions may be attributed to differences in the strength of the metal-carbon monoxide bond in these metal carbonyl derivatives. 

In addition to these two factors which influence the strength of the metal-carbon monoxide bond by clearly affecting the amount of back-bonding or partial double-bonding between the metal atom and the carbon monoxide ligand, there are two other factors which affect the strength of the metal-carbon monoxide bond. 

Comparison between various coordination numbers. The metal-carbon monoxide bonds in tetracoordinated and pentacoordinated metal carbonyls appear to be less stable than the metal-carbon monoxide bonds in the corresponding hexacoordinated metal carbonyls. For example, [Co(CO)4] reacts with allyl halides at room temperature to produce directly the 77-allyl derivative, C3H5Co(CO)3 the presumed pentacoordinate intermediate a-allyl derivative C3H5Co(CO)4 is not isolable, losing carbon monoxide rapidly even at room temperature 110). In contrast to this behavior. 

Mn(CO)5] reacts with allyl halides at room temperature to produce the hexacoordinate a-allyl derivative C3H5Mn(CO)5 which must be heated to 60° C before carbon monoxide is lost at an appreciable rate to form the ir-allyl derivative C3HsMn(CO)4 (99,100). In addition, the pentacoordinate perfluoroacyl derivatives of cobalt, RfCOCo(CO)4 (38, 111, 112), lose carbon monoxide to form RfCo(CO)4 at a lower temperature than the hexacoordinate perfluoroacyl derivatives of manganese, RfCOMn(CO)5 (38,113), form the perfluoroalkyl derivatives, RfMn(CO)s. On the basis of present data, however, it is diflicult to separate the effect of coordination number on the general stability of the molecule from the effect of coordination number on the stability of the metal-carbon monoxide bond. 

Stability of the metal-carbon monoidde bond. A low formal oxidation state, substitution of carbonyl groups by other ligands, and a hexacoordinate central metal atom of either the 3d or the 5d transition series favor a more stable metal-carbon monoxide bond. 

Pentacoordinate HCo(CO)4 is a less stable species than the tetracoordinate [Co(C0)4] , an important factor undoubtedly being the negative charge on the cobalt atom in [Co(CO)4] , which increases the strength of the cobalt-carbon monoxide bonds in the anion. For this reason it is hot at all surprising that HCo(CO)4 is much less stable in hexane solution, where it is present as the undissociated HCo(CO)4, than in aqueous solution, where it is essentially completely ionized into H+ and [Co(CO)4] (23). 

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