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 31 P NMR chemical shifts in the phosphine-substituted metal carbonyls of the type M(CO) 5 PR 3 (M = Cr and Mo; R = H, CH 3, C 6 H 5, F, and Cl) as well as the 95 Mo NMR chemical shift of Mo(CO) 5 P(C 6 H 5 ) 3 and Mo(CO) 5 PX 3 (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 Δδ = δ Μ( CO) 5 PR 3 − δ PR 3 between PR 3 as a ligand, δ M(CO) 5 PR 3, and free PR 3, was analyzed in detail. It was shown that the paramagnetic coupling between the π orbitals of the complexed PR 3 ligand π PR 3 and the d σ metal-based LUMO of the M(CO) 5 PR 3 complex has a positive contribution to the coordination chemical shift, Δδ, whereas the paramagnetic couplings between σ PR 3 and π* PR 3 as well as π PR 3 and π* PR 3 of the complexed ligand have a negative contributions to Δδ for PF 3 and PCl 3 . It is the latter type of couplings that are responsible for the total negative coordination shift in the case of PCl 3 . The calculated 95 Mo NMR chemical shifts of Mo(CO) 5 P(C 6 H 5 ) 3 and Mo(CO) 5 PX 3 (X = F and Cl) are in good agreement with experiment. The major contribution comes from the paramagnetic coupling between the occupied d π orbitals (HOMO) and the virtual d σ orbitals (LUMO).