Ligand-Field Theory of Linear Gaseous Molecules Involving the First Transition Series Elements

Abstract

The diatomic and linear triatomic molecules such as the oxides and halides of the ``iron‐group'' elements are of importance in high temperature systems. Although the various high symmetry compounds of these elements, e.g., the solid oxides, hydrated complexes, etc., are sufficiently understood in terms of the present ligand‐field theory, no semiquantitative schemes have been utilized to understand and predict the spectroscopic and thermal properties of their simple, gaseous molecules. In this article, the observed dissociation energy vs atomic number curves for the linear gaseous oxides and the halides of the first transition series are compared with the corresponding energy curves of the solid oxides and the halides, and the similarities are pointed out. It is shown that the double maximum stabilization curves observed in the gaseous molecules as well can be understood in terms of the splitting of the (3d) shell of the transition atoms in a linear (C<sub>∞v</sub> or D<sub>∞h</sub> symmetry) ligand field with two independent splitting parameters, one of which is small. A method for predicting the low‐lying molecular spectral states is developed. The energies of the states depend on the splitting parameters and the electron interactions. The parameters are estimated from the first portion of the thermal data and used only for these elements. The electron interaction energy is obtained from the spectral terms of free atoms (ions). Unlike the case in the high symmetry applications, this procedure here takes into account ``electronic correlation'' (configuration interaction) effects. The generality of the treatment is revealed by comparing the results of the simple ionic and covalent models with those of the molecular orbital method. Calculations of spectral states are made for ScO, TiO, VO, and CrO, and experimental data are discussed.