Unstable intermediates. Part XC. The radicals AlR3–, SiR3, and PR3+: their electron spin resonance spectra and pyramidal character

Abstract

Radicals tentatively identified by their e.s.r. spectra as AlR₃^–(R = alkyl), PH₃⁺, and PR₃⁺ have been prepared by exposure to ⁶⁰Co γ-rays of the corresponding aluminium and phosphorus alkyls, the latter after dissolution in sulphuric acid. These results, together with those for AlH₃^–, SiH₃, and SiR₃ radicals show that the estimated 3p to 3s ratios increase markedly from aluminium to phosphorus, which suggests that these pyramidal radicals flatten in this order, as expected in terms of Pauling's electronegativity theory.

This trend is very much more marked for the alkyl derivatives than for the hydrides, the effect of alkyl substitution being to flatten the aluminium derivatives, but to bend those of phosphorus. This is interpreted in terms of hyperconjugation which increases as the positive charge increases.