Electronic structure of transition-metal hydrides: NiH and PdH

Abstract

Ab initio self-consistent field (SCF), and configuration interaction (CI), calculations have been performed for the diatomic transition metal hydrides NiH and PdH to study the behavior of the d electrons and their effect on chemical bonding. SCF calculations have been carried out for the three low-lying states, Δ2, Π2, and +2, which can be formed with the metal atom in the (d9s1) configuration. The ground state of NiH is Δ2 with the Π2 and +2 states only about 0.1 eV above. In the case of PdH the +2 state is the lowest with the Δ2 about 0.6 eV above. This difference can be attributed to the participation of the 4d electrons in the PdH bond. The 3d electrons in NiH seem to be more localized and "atomic d-like." In both cases the potential energy curves are too broad and the equilibrium bond lengths are longer than those found from spectral data. For both molecules, the CI results are considerably improved over those from the SCF calculations. However, there are still significant differences between the computed (CI) and observed values for the ground-state equilibrium bond length and force constant. Two effects are considered in order to understand these errors. For NiH, the limited CI is unable to properly account for the differential correlation energy of the 3d94s1 and 3d84s2 terms of the free Ni atom and this probably leads to the errors in the calculation. For PdH, differential atomic correlation energies are likely to be much less important, but relativistic effects are likely to be important. We have also investigated some excited states and the calculated vertical term energies are compared with experimental results. © 1981 The American Physical Society.