ELECTRONIC-STRUCTURE AND BONDING IN TETRADECKER SANDWICH COMPLEXES

Electronic structure of tetradecker sandwich compounds, CpMCpM'CpMCp, have been studied by using the fragment moleular orbital approach within the extended Hückel molecular orbital theory. Forty-six valence electron complexes are found to have highly antibonding degenerate HOMOs which lead to a distorted structure where the two end CpMCp units slip away from the central metal in a direction perpendicular to the original metal-metal axis (5). The extent of antibonding interactions is maximum when the M' is a late transition metal and M is an early transition metal of the periodic table. With appropriate metals it is possible to have a symmetric 46 valence electron tetradecker complex. Calculations on a series of CpCo(C3B2H5)M'(C3B2H5)CoCp where M' = Cr, Mn, Fe, Co, Ni, Cu, and Zn reproduced the experimental structural trends. The variation in total energies has been traced predominantly to the energy difference between the valence orbitals of the metal atoms and the π and σ MOs of the ring. Qualitative prediction of the extent of slipping is difficult because of the simultaneous variation of the number of valence electrons and the metal atom parameters. However calculations with appropriate atomic parameters reproduce the experimental trends well. The bent geometry of CpCo(C3B2H5)Sn(C3B2H5)CoCp is caused by the factors similar to those that operate in making CpSnCp a bent metallocene. © 1990, American Chemical Society. All rights reserved.