What Accounts for the Remarkable Difference between Silabenzene and Phosphabenzene in Stability toward Dimerization?

B3LYP/6-31G* calculations have been performed in order to understand the tremendous difference between silabenzene and phosphabenzene in stability toward dimerization. The calculations find that dimerization is exothermic by Δ H = −21.5 kcal/mol for silabenzene but endothermic by Δ H = 32.6 kcal/mol for phosphabenzene. The difference of 54.1 kcal/mol between these enthalpies of dimerization is reproduced to within 11% by twice the difference between the heats of H 2 addition across one of the π bonds between carbon and the heteroatom in each molecule. This finding allows the H 2 addition reactions to be used to analyze the origins of the huge difference between the heats of dimerization. The calculations show that the energy of a π bond to the heteroatom is smaller in silabenzene than in phosphabenzene, but this difference accounts for less than 20% of the difference between the heats of H 2 addition. The major factor is the much greater strength of the σ bond formed to silicon than to phosphorus, and additional calculations have allowed the three contributors to this large difference in σ bond strengths to be identified.