A theoretical study of 31P and 95Mo NMR chemical shifts in M(CO)5PR3 (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 in 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 chemical shift of Mo(CO)(5)P(C6H5)(3) and Mo(CO)(5)PX3 (X = F and Cl). The study was based on density functional theory (DFT) and gauge-including atomic orbitals (GIAO). The calculated chemical shifts and the components of the chemical shift tensor are in good agreement with the available experimental data. The coordination chemical shift expressed as 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 contribution to the coordination chemical shift, Delta delta, whereas the paramagnetic couplings between sigma(PR3) and pi*(PR3) as well as pi(PR3) 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 responsible for the total negative coordination shift in the case of PCl3. The calculated Mo-95 NMR chemical shifts of Mo(CO)(5)P(C6H5)(3) and Mo(CO)(5)PX3 (X = F and Cl) are in good agreement with experiment. The major contribution comes from the paramagnetic coupling between the occupied d(pi) orbitals (HOMO) and the virtual d(sigma) orbitals (LUMO).