Should External Heavy-Atom Effects be Expected in Transition Metal Complexes?

Spin state changes frequently play a key role in the reactivity of transition metal complexes. The rates of spin-forbidden reactions are mediated both by the free energy barrier connecting reactants and products, as well as the strength of spin-orbit coupling (SOC) between the relevant electronic states. Since the 1950’s, there have been numerous demonstrations of external heavy-atom effects on organic molecules, in which a heavy atom, not chemically bonded to the molecule undergoing a change in spin state, perturbs the strength of SOC via an intermolecular effect. However, the potential role of external heavy atom effects on the rates of reactions involving transition metal complexes remains almost entirely unexplored. We report a computational investigation into the changes in SOC that occur along a bimolecular reaction coordinate when an incoming atom coordinates to the prototypical triplet reaction intermediate Fe(CO)4. The calculated changes in SOC are compared for molecules containing atoms ranging in atomic number from Z = 8 to Z = 53 approaching the Fe center (ZFe =26). No evidence for an external heavy atom effect was found, and the changes in SOC with the approach of each incoming group were similar in magnitude. In fact, when taking into account the different minimum energy crossing point geometries for the different incoming groups, the opposite of an external heavy atom effect trend is predicted for this reaction. The results of this computational study suggest that external heavy atom effects are unlikely to have a significant effect on the rates of spin-forbidden reactions for transition metal complexes.