As the calculated energy of distortion to an axial Cas-C angle of 180' is only 1.4 kcal/mol, it is not surprising that sterically encumbered ligands L can overcome this electronic effect and cause axial carbonyls to bend away from L.Other workers have also investigated the reasons why axial carbonyls bend toward ligands that are not ?r ac-ceptors. The seminal work of Elian and Hoffmann on metal carbonyl fragments& addressed distortion of M(C0)4 fragments but did not specifically discuss axial bending. Ziegler and co-workers have published LCAO-HFS calcu-lations@ for HCo(CO), that describe a mixing of the metal dyz orbital with an empty metal 4p, orbital to give a metal dp hybrid which enhances back-bonding to the equatorial carbonyls ( Figure 6). Eisenstein, Crabtree, and co-workers have more recently noted this d-p mixing using EHT.57
ConclusionsWe have confirmed the dominance of metallacyclopropane character in 1 and provided an explanation for the observed distortion of its axial carbonyl ligands. In addition we have documented, by comparison with its X-ray structure, the geometric accuracy of ab initio calculational methods for third row transition metal complexes and have demonstrated the utility of the Krauss-Stevens22 carbon and oxygen effective core potentials. Acknowledgment. (53) Axe, F. U.; Marynick, D. S. J. Am. Chem. SOC. 1984,106,6230. Much earlier theoretical work on 8 is referenced therein. (54) Ref 31 in ref 3 4 (a) +acenaphthalene: Cotton, F. A.; Lahuerta, P. Znorg. Chem. 1975,14,116. (b) $-furanone: Chisnall, B. M.; Green, M.; Hughes, R. P.; Wells, A. J. J. Chem. SOC., Dalton Trans. 1976,1899. (55) Elian, M.; Hoffmann, R. Inorg. Chem. 1975,14,1058. (56) Versluis, L.; Ziegler, T.; Baerends, E. J.; Ravenek, W. J. Am. (57) Jackson, S . A.; Eisenstein, 0.; Martin, J. D.; Albeniz, A. C.; Chem. SOC. 1989,111,2018.
