The electronic structure of substituted 1,2,4,5-tetraphosphaspiro[2.2]pentanes and 1,2,4,5-tetraphospha-3-silaspiro[2.2]pentanes

Abstract

The sequence of the highest occupied molecular orbitals of 1,2,4,5-tetraphosphaspiro[2.2]pentane, its 1,2,4,5-tetramethyl substituted derivative, and the corresponding 3-sila analogues is derived by means of molecular orbital (m.o.) calculations. The calculations (MNDO, STO-3G) reveal a small split of the valence orbitals due to spiro-interaction. The results of the model calculations are in accord with photoelectron (p.e.) spectroscopic investigations on 1,2,4,5-tetra-t-butyl-1,2,4,5tetraphosphaspiro[2.2]pentane (1a) and its 3-sila congener (2a) assuming S₄ symmetry. The assignment of the p.e. bands is based on the comparison between the p.e. spectra of (1a) and (2a) those of 1,2-di-t-butyl-3,3-diphenyl-1,2-diphospha-3-silacyclopropane and 1,2-di-t-butyl-3,3dimethyl-1,2-diphosphirane and on the results of m.o. calculations. Essentially no spiro-interaction could be detected.