Hydrocarbon Activation by Transition-Metal Complexes

The involvement of the triplet state of hydrocarbons is a common feature of spin uncoupling in activation of hydrocarbons by transition metal species. This does, however, not mean that the triplet state of the hydrocarbon is involved in the reaction mechanism at the real kinetic stage (this is close to the case of alkene adsorption on cupper surfaces [54]). Instead it represents a way of analysis of the deformation of the wave function during the catalytic reaction in terms of VB structures. 

The energy of the low-spin d +1s1 state of the TM atomi, found earlier as an important criterion for C-H bond activation [48, 50], can be accounted for by the VBCI treatment as well, since it naturally participates in the Cl mixing of all low-spin states. Thus the VB treatment is compatible with the sd hybridization and unshielding mechanism proposed by Blomberg at el. [48, 50]. 

The account of promotion and exchange-loss energies [50] can not explain all trends in the barrier heights for TM atoms and complexes. A reason for this is 

An important advantage of the VB approach to catalysis is that it is conceptually closely related to the traditional way in which chemists rationalize chemical structure and reactivity [6, 7, 45]. It is of considerable interest to construct diabatic electronic states representing specific resonance structures of a catalytic system. In this way the importance of the triplet excited s-tate of organic substrates becomes immediately evident, and it is then easy to understand that only the low spin state of the catalyst is reactive. 

This work was supported by Swedish Institute and by Swedish Royal Academy of Science. 

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With this well-studied process [15] as a model we apply a simple VB approximation in order to stress spin-uncoupling features which are not seen in a conventional MO approach. We then discuss hydrocarbon activation by transition metal complexes in general terms. 

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