CO Binding Introduction of Backbonding

The bonding of carbon monoxide to a transition metal is more complex than the simple combination of an electron pair and an imoccupied orbital in a Lewis add-base interaction. Carbon monoxide is weakly basic. The formation of HCO does not occur imder standard synthetic conditions. HCO has been invoked as an intermediate in superacid solution and has been identified in interstellar space. Yet CO binds tightly to many transition metals and even binds weakly to simple main group Lewis adds like BH and d° transition metals. 

Orbital interactions that create bonding orbitals for the coordination of CO between two transition 

Much physical evidence reveals the importance of backbonding. For example, the C-O distance in carbonyl ligands is longer than that in free CO, and the carbonyl stretching frequency is lower than that in free CO. Both of these observations result from the decrease in C-O bond order that results from backbonding. 

The value of v for terminal carbonyl ligands on metal fragments with d-electrons is typicaUy between 2125 and 1850 cm , while the value for free CO is 2143 cm . Unusually electron-rich neutral complexes can display even lower values of v. For example, Fe(dmpe)j(CO) has a v. of 1812 cm . A positive charge on a complex decreases back-bonding and increases v, as can be seen by comparing the value of compounds A and 

Comparisons of v q values between compounds with different numbers of carbonyl ligands or different symmetries are not straightforward. The observed fiequencies of vibration 

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