An assessment of the parameters relevant to the subdivision of sigma and pi electronic effects in M-P bonds

The origin of the sigma and pi controversy in inorganic and organometallic chemistry, particularly with respect to M-P bonds, is comprehensively analyzed. It is concluded that traditionally used physical parameters such as v(CO), delta((CO)-C-13), (1)J(M-P), E(o) and r(M-P) of metal carbonyl complexes can not be used to resolve the sigma and pi controversy due to the parallel influence of weak sigma and strong pi bonding. In contrast, the parameters Delta E, bond strength and metal NMR isotropic chemical shift should be useful to distinguish sigma and pi effects because they respond to the weak sigma and strong pi effects in opposite directions. The metal NMR isotropic chemical shift is undoubtedly the best probe due to the high accuracy of experimental measurement. Its application in the separation of sigma, pi and steric effects in M-P bonding should be possible.