Metal ion solvation in the gas phase: The quest for higher oxidation states

The gas-phase study of small metal ion-molecule complexes offers one of the few opportunities in which cluster science and condensed phase behavior can relate at a quantitative level. Until recently, however, there remained a fundamental barrier inhibiting further progress in gas-phase experimentation on metal-ion solvation. The majority of metal ions of chemical and biochemical interest in the condensed phase carry a formal charge greater than +1: a situation which contrasts markedly with the observation that most experiments on gas-phase metal ion-molecule complexes have concentrated on singly charged species. However, during the past five years significant progress has been made to redress this imbalance, with the development of new techniques for generating multiply charged metal-ligand complexes in the gas phase. Despite considerable technical difficulties, two quite separate methods (electrospray and pick-up) have been shown to be capable of producing a wide range of doubly and some triply charged metal-ligand cations and, in the case of electrospray, multiply charged anions as well. This review examines some of the challenges these new experiments have presented and how our view of quite basic processes, such as hydrolysis, may be altered as a result of investigations in the gas phase.