Spectrum-Structure Correlations in Hexacoordinated Transition Metal Complexes

Abstract

To understand the photophysical and photochemical processes a comprehensive knowledge of the molecular energy level scheme is required. It has been found in recent years that even slight changes of the geometric structure can significantly alter the electronic properties of transition metal complexes. This has led to a growing interest in the study of spectrum-structure correlations, in particular for chromium(III) compounds which are of current interest as possible candidates for luminescent solar concentrators (1). In the first section of this paper the potential of common ligand field theory (LFT) will be illustrated to describe trigonal distortions for complexes near to high symmetry. For the properties considered the hexaamminechrornate(III) ion represents an ideal system for the global parametrization within the LFT which is reflected by the high molecular symmetry and the absence of metal-ligand π-bonding. The angular overlap model (AOM), on the other hand, is based on an additive description of metal-ligand interactions using local bonding parameters of σ- and π-type which are more adequate to the chemical way of thinking. One of its advantages over the LFT is that it is easier to include the molecular geometry into energy level calculations, since the AOM parameters are independent of angular distortions within the coordination sphere (2). In the case of osmium(IV) complexes a great deal of information about electronic and geometric structure can be derived from detailed studies of intraconfigurational transitions, and model parameters are given here for the first time. In order to explain spectroscopic properties of transition group metal acetylacetonates an extended AOM is required. Directional π-bonding effects resulting from the phase coupling of chelate molecular orbitals lead to non-additive contributions to the d-orbital energies. We will give an illustration of this type of study below.