Metal compounds

Theoretical spectroscopy of transition metal complexes poses special challenges for quantum chemistry. Relativistic effects lead to spectral shifts of electronic transitions – the yellow color of gold originates from blue light absorption as opposed to absorption of UV light by silver – and to multiplicity mixing which in turn promotes intersystem crossing and phosphorescence. These effects are particularly pronounced in late third-row elements but cannot be neglected in the modeling of lighter transition metal compounds either. Differential electron correlation effects, on the other hand, are particularly large in metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer states (LMCT) states of first-row transition metal complexes due to their compact d shells. Additional complications are the multiconfigurational character of the excited states, where ligand-centered (LC) and metal-centered (MC) transitions are mixed with MLCT, LMCT and possibly ligand-to-ligand charge transfer (LLCT) transitions, and the open-shell structure of many transition metal complexes in the electronic ground state.