Two‐State Reactivity in Organometallic Gas‐Phase Ion Chemistry

In contrast to organic reactions, which can almost always be described in terms of a single multiplicity, in organometallic systems, quite often more than one state may be involved. The phenomenon of two states of different multiplicities that determine the minimum‐energy pathway of a reaction is classified as two‐state reactivity (TSR). As an example, the ion/molecule reactions of ‘bare’ transition‐metal‐monoxide cations with dihydrogen and hydrocarbons have been analyzed in terms of the corresponding potential‐energy hypersurfaces. It turns out that, besides classical factors, such as the barrier heights, the spin‐orbit coupling factor is essential, since curve crossing between the high‐ and low‐spin states constitutes a distinct mechanistic step along the reaction coordinates. Thus, TSR may evolve as a new paradigm for describing the chemistry of coordinatively unsaturated transition‐metal complexes. This concept may contribute to the understanding of organometallic chemistry in general and for the development of oxidation catalysts in particular.