A reconsideration of the role of slippage in the activation of co-ordinated olefins towards nucleophilic attack

Abstract

Activation with respect to attack by hydride ion of the co-ordinated olefins in the complexes [Fe(cp)(CO)₂(olefin)]⁺(olefin = ethylene or propene; cp =η⁵-C₅H₅) has been investigated theoretically utilizing extended-Hückel and INDO methodologies. Extended-Hückel molecular orbital (EHMO) calculations suggest (a) that the receptor orbital of the ethylene complex, of predominantly olefin π* character, is destabilized relative to the corresponding orbital of the free ligand, and (b) that nucleophilic attack can only be rationalized by invoking distortion (slippage or tilting) of the co-ordinated olefin during attack. These results are consistent with previous work. In contrast, INDO and (indirectly)ab initio calculations suggest that the same receptor orbital is stabilized on co-ordination, in effect resulting in activation of the olefin towards nucleophilic attack without the necessity of invoking distortion, although distortion does result in increased activation. Both the EHMO and INDO calculations suggest that the lowest unoccupid molecular orbital of the ethylene complex is of predominantly CO π* rather than the olefin π* character. In addition, both methodologies suggest that there is greater positive charge on the CO than on the ethylene carbon atoms, and thus it would seem that hydride attack should occur at CO rather than at the olefin.