Ligand field spectroscopy of Cu(II) and Ag(II) complexes in the gas phase: theory and experiment

Ligand field spectra have been recorded in the gas phase for the two series of complexes containing either Cu(II) or Ag(II) in association with pyridine. Where comparisons are possible, the gas phase spectra match those recorded in the condensed phase; however, for Ag(II) systems the results differ in interpretation. The Ag(II) data are attributed to a ligand-to-metal charge transfer process, and the Cu(II) data (spectral region and extinction coefficient) match the characteristics of a d-d transition. A detailed theoretical analysis of two complexes, [Cu(py)4]2+ and [Ag(py)4]2+ provides evidence of a minimum energy, D 4h structure and two less stable D2h and D2d structures within ~60 kJ mol-1. From these structures it is possible to identify a range of optically and vibronically allowed transitions that could contribute to spectra observed in the gas phase. In the case of calculations on [Ag(py)4]2+ there is strong evidence of an electronic transition that would account for the observation of charge transfer in the experiments. Less detailed calculations on [Cu(py) 6]2+ and [Ag(py)6]2+ show structural evidence of extensive Jahn-Teller distortion. Taken together with incremental binding energies calculated for complexes containing between two and six pyridine molecules, these results show that the level of theory adopted is capable of providing a semi-quantitative understanding of the experimental data.