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 tr emendous difference between silabenzene and phosphabenzene in stability tow ard 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 differe nce between the heats of H-2 addition across one of the pi bonds between ca rbon and the heteroatom in each molecule. This finding allows the H-2 addit ion reactions to be used to analyze the origins of the huge difference betw een the heats of dimerization. The calculations show that the energy of a p i bond to the heteroatom is smaller in silabenzene than in phosphabenzene, but this difference accounts for less than 20% of the difference between th e heats of H-2 addition. The major factor is the much greater strength of t he sigma bond formed to silicon than to phosphorus, and additional calculat ions have allowed the three contributors to this large difference in sigma bond strengths to be identified.