METAL-LIGAND BONDING IN LM(CO)(5) COMPLEXES (L = NME3, PME3, PCL3, PBR3 OR ASME3, M = CR, MO OR W) FROM DIELECTRIC, ELECTROOPTIC AND SPECTROSCOPIC EVIDENCE

A comparative study has been made of the electric dipole moments, optical polarisability anisotropies (from electric birefringence measurements), infrared carbonyl stretching frequencies and C-13 NMR spectral shifts of LM(CO)5 complexes where L = NMe3, PMe3, PCl3, PBr3 or AsMe3 and M = Cr, Mo or W. Analysis of the polarisability anisotropies of the complexes provides direct, experimentally based evidence indicating a substantial pi-component in the phosphorus-metal and arsenic-metal bonding in P(As)Me3M(CO)5 within a highly polarisable P(As)-M-(trans)CO delocalised pi-electron system. The L-M pi-bonding appears not to be significantly affected by change of L from PMe3 to AsMe3 or by change of M within the chromium triad. It is shown that pi-bonding between the phosphorus and the metal and electron delocalisation along the P-M-(trans)CO linkage are enhanced by electronegative halogen substituents in the phosphine ligand. The conclusions reached are in general accord with results from interpretation of the infrared and C-13 NMR spectral and dipole moment data.