A THEORETICAL-STUDY OF P-31 AND MO-95 NMR CHEMICAL-SHIFTS IN M(CO)(5)PR3 (M = CR, MO R = H, CH3, C6H5, F, AND CL) BASED ON DENSITY-FUNCTIONAL THEORY AND GAUGE-INCLUDING ATOMIC ORBITALS

A theoretical study has been carried out on P-31 NMR chemical shifts i n the phosphine-substituted metal carbonyls of the type M(CO)(5)PR3 (M = Cr and Mo; R = H, CH3, C6H5, F, and Cl) as well as the Mo-95 NMR ch emical shift of Mo(CO)(5)P(C6H5)(3) and Mo(CO)(5)PX3 (X = F and Cl). T he study was based on density functional theory (DFT) and gauge-includ ing atomic orbitals (GIAO). The calculated chemical shifts and the com ponents of the chemical shift tensor are in good agreement with the av ailable experimental data. The coordination chemical shift expressed a s the difference in the isotropic shifts Delta delta = delta(M(CO)5PR3 ) - delta(PR3) between PR3 as a ligand, delta(M(CO)5PR3), and free PR3 , was analyzed in detail. It was shown that the paramagnetic coupling between the pi orbitals of the complexed PR3 ligand pi(PR3) and the d( sigma) metal-based LUMO of the M(CO)(5)PR3 complex has a positive cont ribution to the coordination chemical shift, Delta delta, whereas the paramagnetic couplings between sigma(PR3) and pi(PR3) as well as pi(P R3) and pi(PR3) of the complexed ligand have a negative contributions to as for PF3 and PCl3. It is the latter type of couplings that are r esponsible for the total negative coordination shift in the case of PC l3. The calculated Mo-95 NMR chemical shifts of Mo(CO)(5)P(C6H5)(3) an d Mo(CO)(5)PX3 (X = F and Cl) are in good agreement with experiment. T he major contribution comes from the paramagnetic coupling between the occupied d(pi) orbitals (HOMO) and the virtual d(sigma) orbitals (LUM O).