The electronic structure of 'linear' nickel oxides

The geometries, ground states and low-lying excited states of the 'linear' nickel oxides NiO, [NiO2](2-) and [NiO2](3-) have been studied using local density-functional (LDF) calculations within the linear combination of Gaussian-type orbitals framework. The NiO molecule and the [NiO2](2-) ion are predicted to have (3) Sigma(-) and (3) Sigma(g)(-) ground states respectively. The calculated bond lengths and vibrational frequencies are in excellent agreement with experiment. Cellular ligand-field (CLF) analyses of the spectra and magnetic properties have also been performed. The LDF and CLF calculations provide a consistent description of the bonding and suggest that the oxide ions are acting as good sigma and pi donors in these molecules with both roles enhanced compared to systems with higher co-ordination numbers. The interaction between the d(sigma) orbital and the antibonding sigma orbital of predominantly nickel s-orbital character is important in understanding the electronic structure of these systems and its modelling in CLF analyses is described. The description of the bonding in NiO is consistent with that obtained from previous studies. The prediction of a (3) Sigma(g)(-) ground state for the [NiO2](2-) ion required a reassignment of the spectrum and led to a more satisfactory modelling of the magnetic moment than that given by earlier calculations. The ground state of the [NiO2](3-) ion is predicted to be (2) Sigma(g)(+), consistent with its EPR spectrum. The bonding in this low-oxidation-state is weaker than in the nickel(II) oxides and involves considerable contributions from both nickel d and s orbitals.