Ab initiocluster-model study of the electronic ground-state and photoemission properties ofNiN2and NiCO: Models for chemisorption

The ground state of the linear ${\mathrm{NiN}}_2$ cluster is found, from generalized valence-bond configuration-interaction calculations, to be a ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state with a short Ni–N distance of 1.64 Å and a Ni–${\mathrm{N}}_2$ dissociation energy of 0.8 eV. In this ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state near the calculated equilibrium internuclear distance, the electronic wave function may be characterized as arising from a significant admixture of the Ni 3$d^{10}$ configuration, and the bonding of the ${\mathrm{N}}_2$ molecule to the Ni atom may be characterized as ${\mathrm{N}}_2$ σ donation and Ni 3dπ backward donation. Low-lying excited states ${(\mathrm{}}^3$ ${\Sigma }^{+}$, ${}_{}{}^3{}_{}{}^{}\mathrm{\Delta }$, and ${}_{}{}^3{}_{}{}^{})$ arising from the Ni 3$d^9$4$s^1$ configuration are also studied. Mulliken population analyses, dipole moments, and vibrational and photoemission properties are calculated. Parallel calculations are also performed for the linear NiCO molecule. The ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ states of both ${\mathrm{NiN}}_2$ and NiCO are good models for the chemisorption of ${\mathrm{N}}_2$ and CO on a Ni surface, with regard to the downward shift of the molecular stretching frequency in the interacting systems, the photoemission satellite structure, and the orientation of the dipole moments (with a net electron transfer from Ni to ${\mathrm{N}}_2$ or CO).