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.
Introduction. -The role of concepts which can provide new insight in chemical problems can hardly be overemphasized, and the present article describes the two-statereactivity (TSR) paradigm and its manifestation in H-H and C-H bond activation by 'bare' 0x0-cations MO+ of the late, first-row transition metals Mn-Cu
With a few notable exceptions, for example 0, and CH,, organic species generally possess low-spin ground states, and their reactions proceed on a single potential energy surface; this will be referred to as single-state reactivity (SSR). To conceptualize the key features of SSR, it is in principle sufficient to understand the transition structures (TSs), their entropic requirements, and the associated barrier heights. Indeed, the arsenal of reactivity paradigms that has evolved in organic chemistry, such as linear free energy relationships [3], frontier molecular orbital theory [4], the Woodward-Hoffmann rules [5], and the valence bond (VB) crossing diagrams [6], all are related to SSR. Thus, our intuition derives from the experience with surfaces of a single multiplicity. In fact, even photochemical reactivity in the sense of cleavage and formation of chemical bonds is basically treated on a single (though excited) potential-energy hypersurface. Nevertheless, it is clear that SSR is only one aspect in chemical reactivity.
Coordinatively unsaturated transition-metal compounds often possess high-spin ground states and nearby low-spin excited states [7-91. As a result of the adjacency of these spin states, the reactivity of these compounds may generally involve (at least) two states in which the ground state must not necessarily be the most reactive one. TSR is characterized by a crossing of two potential-energy hypersurfaces of different multiplic-