Electronic state effects in sigma bond activation by first row transition metal ions: the ion chromatography technique

Abstract

The ability of transition metal ions to activate C—H and C—C bonds of small saturated hydrocarbons has been attributed to the high density of lowlying excited states available to the transition metal center. The details of the interactions of these metal ion states with the reactant molecule, however, are not well understood and the desire to understand these details has stimulated significant experimental and theoretical interest. On the experimental side, gas-phase transition metal ion chemistry has been explored extensively [1–3] and recently state specific studies have been carried out [4–17]. Theoretically, ab initio electronic structure calculations provided insight regarding the nature of the bonding and trends in bond energies [18–25]. In addition, ab initio potential energy surfaces allow chemists to explore the interaction of the low-lying excited states of metal ions with reactant molecules [26]. Both theory and experiment indicate the reactivity of a given state of the metal ion will depend on its interaction with other nearby electronic states. Surface crossings are commonplace and “spin-forbidden” reactions are often observed for transition metal ions (spin is generally conserved for bimolecular reactions involving light elements, but only the total angular momentum must be rigorously conserved for heavier elements) [27]. Because reactions involving atomic transition metal ions are inherently complex, quantitative state specific experimental studies along with theoretical calculations are necessary to understand their reaction energetics and mechanisms.