Recent advances in the visible and UV spectroscopy of metal dication complexes

Experimental techniques developed during the past 15 years have demonstrated that it is possible to prepare stable metal dication complexes, such as [Cu(NH3)N]2+ and [Ni(H2O)N]2+, in the gas phase. The significance of these complexes lies in the fact that they contain metal ions that are in their more common charge state, and therefore, a link between their properties and those found for the same ions in the condensed phase is readily accessible. In this review, we focus on one aspect of the study of these ion complexes, and that is their visible and ultraviolet (UV) spectroscopy. Current experimental techniques for recording spectra in the gas phase are discussed together with the theoretical methods required to interpret the spectra of metal dication complexes. An attempt is made to identify any barriers that might exist to measuring the optical spectra of metal dication complexes using current ion beam technology, where a typical experiment will involve measuring either photofragment yield or ion beam depletion as a function of laser wavelength. One very obvious area of spectroscopy to be explored, and one that is unique to transition metal complexes, is ligand field spectroscopy. Estimates of photofragment yields based on typical absorption cross sections and the kinetics of fragmentation highlight the difficulties involved in measuring such spectra. Of the theoretical techniques currently available for calculating spectral transitions, the method most commonly used for metal complexes is time-dependent density functional theory (TDDFT). Using selected examples, it is shown that although TDDFT is, for the most part satisfactory, extreme caution should be exercised when investigating the electronic states of open-shell complexes. An obvious conclusion to emerge is that a theoretical method that predicts the correct ground state geometry of an open-shell complex (and is free from spin contamination) does not necessarily yield the correct electronically excited states due to multi-electron character and/or spin contamination in the excited state manifold. It is anticipated that the development of experimental techniques that can record accurate electronic spectra will provide new and more demanding benchmarks for the refinement of theoretical methods.