α-stabilization by silyl and phosphino substitution

The electron affinity of the bis(dimethylphosphino)methyl radical was measured to be 35.3 +/- 0.2 kcal/mol, using electron photodetachment spectroscopy in an ion cyclotron resonance spectrometer. Using equilibrium measurements, Delta H(acid)degrees of bis(dimethylphosphino)methane and bis(trimethylsilyl)methane was determined to be 370 +/- 3 and 373 +/- 3 kcal/mol, respectively. From measured and known electron affinities and gas-phase acidities, we derive C-H bond dissociation energies: bis(dimethylphosphino)methane, 92 +/- 3 kcal/mol, and bis(trimethylsilyl)methane, 95 +/- 3 kcal/mol. Delta H(acid)degrees of trimethylphosphine was bracketed al 383-387 kcal/mol. The alpha-stabilization effect of silyl and phosphino substitution is large and comparable in size to stabilization by thio and chloro substitution. Possible mechanisms of stabilization are discussed.