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, 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. of trimethylphosphine was bracketed at 383−387 kcal/mol. The α-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.