INTERRUPTION OF CONJUGATION IN POLYENES BOUND TO TRANSITION-METAL FRAGMENTS
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In the past polyenes bound to transition-metal fragments were usually found to possess equal C-C bond lengths within experimental error. Recently, accurate studies of (6-C6H6)Cr(CO)3, (5-C5H5)Mn(CO)3, and (5-C5H5)Co(CO)2 show that there is a pattern of short and long C-C bonds in these metal-bound polyenes. We have calculated the molecular orbitals of a large number of molecules in the series (n-CnHn)M(CO)m(XY) (n = 3-7, m = 1-3, XY = CO, N2, NO, CN). Density difference plots show that the origin of these patterns in the C-C bond lengths is an intramolecular interruption of the conjugated system of the polyene. The calculations suggest a general rule that C-C bonds in a polyene eclipsed below by a carbonyl or similar ligand will be long while those trans to these ligands will be short. These results are in agreement with recent accurate structures and even with some older structures where the significance of the differences in bond length is questionable. Exceptions to this rule occur for some room-temperature crystal structures in which the C-C distances are not precisely determined and for some systems in which there is a strong specific intermolecular interaction. 1983, American Chemical Society. All rights reserved.