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 Delta H = -21.5 kcal/mol for silabenzene but endothermic by Delta 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 pi 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 pi 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 sigma bond formed to silicon than to phosphorus, and additional calculations have allowed the three contributors to this large difference in sigma bond strengths to be identified.