LINEAR SEMIBRIDGING CARBONYLS .4. A CONSEQUENCE OF STERIC CROWDING AND STRONG METAL-TO-METAL BONDING
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Ab initio full-gradient geometry optimizations have been performed on 28 of 36 M-M closed-shell electronic configurations for the M2(CO)2 (M = Sc and Mn) model systems. Those configurations whose energy favors a bridging carbonyl structure always prefer a bent semibridging or symmetrically bridging geometry. Conversely, when the carbonyls, of those configurations whose energy favors terminal carbonyls, are forced into bridging sites, a linear semibridging structure results. Thus, linear semibridging carbonyls are a consequence of steric crowding. Generally, those configurations that favor a bent semibridging or symmetrically bridging carbonyl have more metal-to-metal antibonding molecular orbitals occupied (particularly the *). In contrast, those configurations that favor a terminal structure, and hence a linear semibridging carbonyl in sterically crowded molecules, have more metal-to-metal bonding orbitals occupied. Thus, the appearance of linear semibridging carbonyls signifies strong metal-to-metal bonding. 1992, American Chemical Society. All rights reserved.