The role of triplet excited state of hydrocarbons in catalysis by transition-metal species

Reactions of the second-row transition metal (TM) atoms, clusters and complexes with small hydrocarbons have been considered by a simple valence bond (VB) approximation together with conventional molecular orbital approaches. Correlation diagram obtained from VB treatment of the C-H bond activation by TM atom demonstrates the involvement of the triplet excited state of hydrocarbon into the total wave function at the transition state of the catalytic process. VB approximation indicates that the activation barrier for TM atom insertion into C-H bond of a number of alkanes and alkenes is strongly determined by the "singlet-triplet" avoided crossing when the total spin of the reacting system is fixed by the low-spin term of the TM-catalyst. The energy of the triplet state of hydrocarbon determines the barrier in a simple VB approach. The same general features of spin uncoupling are typical of other TM-species and catalytic processes. The nature of barrier in chemical reaction between two closed shell reagents is often determined by avoided grossing of the ground and doubly-excited singlet states; the latter consists of two triplet excitations coupled to the singlet pairing. In order to diminish the barrier one has to activate in some way the triplet state of the reactants. The catalyst supplies its nonpaired electrons in order to activate the "singlet-triplet" avoided crossing in hydrocarbon.