The potential energy surfaces for the cycloaddition reactions of 16 valence-electron trimetallaallene species have been studied using density functional theory (B3LYP/LANL2DZ). Seven trimetallaallene species of the form MMM with five-membered-rings where M = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, the alkene cycloaddition has been used to study the chemical reactivities of these 16 valence-electron trimetallaallene species. The present theoretical investigations suggest that their relative reactivity increases in the order: CCC < SiSiSi < SiGeSi < GeSiGe < GeGeGe < SnSnSn < PbPbPb. That is, less electronegative and heavier main group atoms will lead to a smaller ΔE_st and, in turn, will facilitate the [3 + 2] cycloaddition reactions to alkenes. Furthermore, the singlet–triplet energy splitting of the 16 valence-electron trimetallaallene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.