The potential energy surfaces for the cycloaddition reaction Me2X: + C-60 (->) Me2X(C-60) (X = C, Si, Ge, Sn, and Pb) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that there are two competing pathways in these reactions, which can be classified as a [6,5]-attack (path 1) and a [6,6]-attack (path 2). It was found that, given the same reaction conditions, the cycloaddition reaction of C-60 via a [6,6]-attack is more favorable than that via a [6,5]-attack, both kinetically and thermodynamically. A qualitative model that is based on the theory of Pross and Shaik has been used to develop an explanation for the reaction barrier heights. As a result, our theoretical investigations suggest that the singlet-triplet splitting Delta E-st(=E-triplet - E-singlet) of the 6 valence electron Me2X: and C-60 species can be used as a guide to predict their reactivity toward cycloaddition reactions. Our model results demonstrate that the reactivity of heavy carbene cycloaddition to C-60 decreases in the order Me2C: > Me2Si: > Me2Ge > Me2Sn: > Me2Pb:. As a consequence, we show that electronic effects play a decisive role in determining the energy barriers as well as the reaction enthalpy.