Structure and bonding in metallocenes and oligomers

Abstract

The structural variations observed as a function of the number of valence electrons, the metals and the rings present in tripledecker and tetradecker sandwiches have been studied by the extended Huckel method. In tripledecker complexes the electron count varies from 26 electrons to 34 electrons depending on the magnitude of the interactions (8a e). With the njckel complex, CpNiCpNiCp⁺¹, the e'₁ antibonding interaction, (8c), is not very high so that the presence of electrons in these orbitals do not affect the stability of the complex. With the vanadium complex, CpVC₆H₆VCp, the e'₁, interaction (8c), is substantial so that 30-valence electron complexes are preferred with the transition metals on the left side of the periodic table. With P₆, P₅ and As₅ complexes the "a₂, (8e), also becomes very strong. This leads to a 28-valence electron closed shell structure. The distortion of the As₅ ring in CpMoAs₅MoCp is attributed to the presence of three electrons in the degenerate HOMOs. In tetradecker complexes 46-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 (7). 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. The bent geometry of CpCo(C₃B₂H₅)Sn(C₃B₂H₅)CoCp is caused by factors similar to those which operate in making CpSnCp a bent metallocene.