The potential energy surfaces for the reactions of phosphino dimetalalkenes featuring an E=E double bond, Rea-E=E, where E = group 14 elements, were investigated using density functional theory (B3LYP/LANL2DZ). Three types of chemical reactions (i.e., the rearrangement reaction, the transition metal complexation reaction, and the [2 + 2] cycloaddition with a diazene) were used to study the reactivity of the Rea-E=E molecules. The theoretical findings reveal that the smaller the singlet triplet splitting (Delta E-st) of the Rea-E=E, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions. Theoretical observations suggest that the relative reactivity increases in the order Rea-C=C < Rea-Si=Si < Rea-Ge=Ge < Rea-Sn=Sn < Rea-Pb=Pb. In other words, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its phosphino Rea-E=E to chemical reaction. It is thus predicted that the phosphino Rea-C=C and Rea-Si=Si molecules should be stable and readily synthesized and isolated at room temperature, since they are quite inert to chemical reaction. The computational results are in good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.