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.