Effects of substituants on the stability of phosphoranyl radicals

The effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation energies of •P(CH ₃)₃X (X = CH₃, SCH₃, OCH ₃, OH, CN, CF₃, Ph) were studied via high-level ab initio molecular orbital calculations. Two alternative definitions for the radical stabilization energy (RSE) were considered: the standard RSE, in which radical stability is measured relative to H-P(CH₃)₃X, and a new definition, the α-RSE, which measures stability relative to P(CH ₃)₂X. We show that these alternative definitions yield almost diametrically opposed trends; we argue that α-RSE provides a reasonable qualitative measure of relative radical stability, while the standard RSE qualitatively reflects the relative strength of the P - H bonds in the corresponding H - P(CH3)3X phosphines. The •P(CH₃)₃X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3pα-type orbital (that occupies the position of the "fifth ligand"), and the σ* orbitals of the axial bonds. Consistent with this picture, the radical is stabilized by resonance (along the axial bonds) with configurations such as X^- P^•+(CH₃)₃ and X• P(CH₃)₃. As a result, substituents that are strong σ-acceptors (such as F, OH, or OCH₃) or have weak P - X bonds (such as SCH₃) stabilize these configurations, resulting in the largest apicophilicities and α-RSEs. Unsaturated π-acceptor substituents (such as phenyl or CN) are weakly stabilizing and interact with the 3pσ-type orbital via a through-space effect. As part of this work, we challenge the notion that phosphorus-centered radicals are more stable than carbon-centered radicals.