A simple valence bond (VB) method is used in order to stress the role of triplet excited state of hydrocarbons in spin-uncoupling mechanism of their activation by transition metal center. The nature of activation barriers in bimolecular reactions is determined by exchange repulsion between closed shell molecules; a catalyst can diminish the barrier assisting the change of spin-coupling schemes in molecules. Involvement of the triplet excited state of activated molecule with participation of open-shelf electrons of the catalyst in configuration interaction scheme is a general requirement for VB interpretation of catalysis. A well-known "donation and back-donation" scheme of molecular orbital description is also compatible with the spin-uncoupling mechanism. Both, the high-spin and low-spin states of a catalyst, are important in CI mixing, but only the low-spin state is reactive.