Periodic trends in the diatomic monoxides and monosulfides of the 3d transition metals

Non-local, density functional calculations have been performed on the ground and low-lying excited states of the 3d transition metal monoxide and monosulfide molecules. Periodic trends in properties such as bond lengths, bond energies, vibrational force constants and dipole moments are examined. The variations in the bond lengths and energies are compared to those in the monoxide and monosulfide solids. Analysis of the electronic structures of the molecules reveals the d-orbital splitting to be d(pi)> d(sigma)greater than or equal to d(delta) and shows the non-bonding role of the d(sigma) and d(delta) functions. This sequence mirrors that in the electronically related dichloride molecules and leads to similarities in the periodic trends with those in the solids. Systematic errors in the calculated bond energies and vibrational frequencies are identified. The successful use of a correctly parameterized ligand-field model is reported allowing quantitative reproduction of the 'd-d' spectra of the monoxide molecules and the prediction of band positions of unassigned transitions.