Computational study of cycloaddition reactions of 16-electron d8 ML4 complexes with C60

The potential energy surfaces of the cycloaddition reactions M(CO)(4) + C(60) → (CO)(4)M(C(60)) (M = Fe, Ru, and Os) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that these reactions have two competing pathways, which can be classified as a [6,5]-attack (path A) and a [6,6]-attack (path B). Our B3LYP results suggest that, given the same reaction conditions, the [6,6]-attack is more favorable than the [6,5]-attack both kinetically and thermodynamically. A qualitative model based on the theory of Pross and Shaik has been used to develop an explanation for the barrier heights. As a consequence, the theoretical findings indicate that the singlet-triplet splitting ΔE(st) (=E(triplet) - E(singlet)) of the 16-electron d(8) M(CO)(4) and C(60) species can be used as a guide to predict their reactivity toward cycloaddition. Our computational results reveal that the reactivity of d(8) M(CO)(4) cycloaddition to C(60) decreases in the order Fe(CO)(4) > Os(CO)(4) > Ru(CO)(4). Accordingly, we demonstrate that both electronic and geometric effects play a crucial role in determining the energy barriers as well as the reaction enthalpy.