C-13- and P-31-NMR study of tetracarbonylbis(diphenylphosphino)alkanemetal(0) complexes of the group 6 elements

Tetracarbonylbis(diphenylphosphino)alkanemetal(0) complexes of the general formula M(CO)(4)[(C-6 H-5)(2) P (CH2)(n) P(C6H5)(2)] (M: Cr, Mo, W; n: 1, 2 , 3) were synthesized from the reaction of M(CO)(4) (eta(2:2)-1,5-cycloocta diene) with the appropriate ligand and characterized by means of IR and NMR (C-13 and P-31) spectroscopy Spectroscopic data shows that two phosphorus atoms in the complexes are identical and occupy two cis-coordiantion sites in a pseudo-octahedral coordination sphere of the metal. The comparative st udy of the coordination shift and the P-31-C-13 coupling constant reveals t hat two carbonyl groups trans to one of two phosphorus atoms in the diphosp hine ligand are shifted toward the lower magnetic field more than those whi ch are cis to both phosphorus atoms. This is ascribed to the fact that two multiface ligands such as carbonyl or phosphine compete for the use of meta l d(pi)-orbitals more strongly when they are trans to each other.