STRUCTURAL SYSTEMATICS - ROLE OF P-A SIGMA-STAR ORBITALS IN METAL-PHOSPHORUS PI-BONDING IN REDOX-RELATED PAIRS OF M-PA3 COMPLEXES (A = R, AR, OR, R = ALKYL)

The geometry variations in the M-PA3 units of 24 transition metal-phosphine and -phosphite complexes whose crystal structures are known for multiple oxidation states of the complex are tabulated and discussed. Conclusions are drawn with regard to the effect of oxidation at the metal on M-P π-back-bonding and the consequent structural effects on the PA3 ligand. Metal-phosphorus bond lengths increase on oxidizing the metal, consistent with the presence of an important element of M-P π-back-bonding. Reduction of M-P π bonding causes a decrease in the average P-A bond lengths, in accord with the PA3 π-acceptor orbital having P-A σ* character. A qualitative molecular orbital theory is described that allows rationalization of these changes and others in PA3 geometry. In particular the effect of M-P π-bonding is to enhance the tendency for a pyramidal PA3 structure, with A-P-A angles ≪120°. © 1990, American Chemical Society. All rights reserved.