Density functional results on the [2+3] cycloaddition of ethylene to various transition metal complexes MO3q and LMO3q (q = -1, 0, 1) with M = Mo, W, Mn, Tc, Re, and Os and various ligands L = Cp, CH3, Cl, and O show that the corresponding activation barriers DeltaE double dagger depend in quadratic fashion on the reaction energies DeltaE(0) as predicted by Marcus theory. A thermoneutral reaction is characterized by the intrinsic reaction barrier DeltaW(0)(double dagger) of 25.1 kcal/mol. Both ethylene: [2+3] cycloaddition to an oxo complex and the corresponding homolytic M-O bond dissociation are controlled by the reducibility of the transition metal center. Indeed, from the easily calculated M-O bond dissociation energy of the oxo complex one can predict the reaction energy DeltaE(0) and hence, by Marcus theory, the cd;responding activation barrier DeltaE double dagger. This allows a systematic representation of more than 25 barriers of [2+3] cycloaddition reactions that range from 5 tb 70 kcal/mol.