Reversible valence equilibrium reactions in main group compounds. A theoretical study

The potential energy surface for the intramolecular reaction of singlet state RR'E=ERR' (E = C, Si, Ge, Sn, and Pb) has been explored using density functional theory. All the stationary points, including the unsymmetrical reactant (R'R(2)E-ER'), the transition state, the symmetric product (R'RE=ERR'), and the monomer (R'RE) were completely optimized at the B3LYP/LANL2DZdp level of theory. Our theoretical findings suggest the following: (1) Both double-bonded RR'C=CRR' and RR'Si=SiRR' species are true minima on their potential energy surfaces and should be the only compounds existing at all temperatures. (2) The germanium system will occur either in the dimeric R(2)R'Ge-GeR' and RR'Ge=GeRR' structures or the monomeric RR'Ge structure, depending on the temperature. (3) If the size of the substituent (R) is small, then the unsymmetrical single-bonded R(2)R'Sn-SnR' molecule can exist at low temperatures. At room temperature, the unsymmetrical R(2)R'Sn-SnR' species can exist in equilibrium with its RR'Sn monomer. (4) The unsymmetrical R(3)Pb-PbR compound may be kinetically stable at low temperatures. On the other hand, it is predicted that both the unsymmetrical R(3)Pb-PbR and the symmetric R(2)Pb=PbR(2) species will spontaneously dissociate into R(2)Pb monomers at room temperature. Our theoretical results are in good agreement with available experimental observations (J. Am. Chem. Soc. 2003, 125, 7520), and the results obtained allow a number of predictions to be made.