The geometry of TiH^2-₆ and VH^-₆

The geometries and relative stabilities of TiH^2-₆ with O_h and D_3h symmetry and of VH^-₆ with O_h, D_3h and C_3v symmetry are calculated with ab initio molecular orbital methods using effective core potentials for Ti and V. With inclusion of electron correlation using quadratic CI (QCISD(T)) and coupled cluster (CCSD(T)) theory the D_3h form of TiH^2-₆ is predicted to be slightly (≈3 kcal/mol) lower in energy than the octahedral form. At the same level of theory the C_3v form of VH^-₆ is calculated as ≈50 kcal/mol more stable than the O_h form and ≈3 kcal/mol more stable than the D_3h form. Energy calculations using Møller-Plesset perturbation theory show strongly oscillating results at the MP2, MP3 and MP4 level and thus, are not reliable for estimating relative energies of transition metal complexes.