PSEUDO 2ND-ORDER JAHN-TELLER EFFECTS AND SYMMETRY CONSIDERATIONS IN TRANSITION-METAL POLYHYDRIDE COMPLEXES

Pseudo second-order Jahn-Teller distortions are proposed to explain wh y the equatorial hydride ligands in a series of cyclopentadienyl metal polyhydrides (CpReH6, Cp*OsH5, Cp*IrH4) bend away from the Cp ring a t such large angles. Restricted Hartree-Fock (RHF) geometry optimizati ons on models of these complexes where the Cp ring has been replaced by a hydride reproduce these large distortions. Walsh diagrams and Mul liken populations have been used to show that the stability provided b y distortion results from maximizing the overlap population between th e hydride s orbitals and the metal d orbitals. This study also include s a comparison of previous work on the structures of d(0) MH6 complexe s with our work on d(2) MH6 complexes. We found that an occupied d orb ital has a significant effect on the geometry of this type of molecule due to the change in hybridization on the metal from sd(5) to spd(4). Symmetry arguments are demonstrated as a means to predict the low ene rgy geometries of these complexes.