A density functional study of nuclear magnetic resonance spin-spin coupling constants in transition-metal systems

We report the first extensive DFT-based study of the one-bond metal-ligand nuclear spin-spin coupling constant K-1(M, X). Calculations are presented on K-1(M,X) in 3d-, 4d- and Sd-transition-metal carbonyl, ore, fluoro, and phosphine complexes of affordable size for which experimental coupling constants are available. An analysis based on molecular orbitals makes it possible to identify all key contributions to the Fermi-contact (FC) coupling term and rationalize the periodic trends in the coupling constants. Based on this analysis, it is shown that the FC coupling contribution is a valence property with core orbitals playing only a minor, indirect role. Through a similar analysis it is concluded that lone pair orbitals hardly contribute to the paramagnetic spin-orbit (PSO) coupling term. The study also examines the validity of the finite perturbation method (for the FC term) and the uncoupled perturbation method (for the PSO term). The results from frozen-core calculations in 3d- and 4d-complexes are mostly in good agreement with experimental data. The quasirelativistic correction on top of the frozen-core treatment gives a significant but insufficient improvement for the coupling constants in Sd-complexes. The impact of relativity on the size of K-1(M,X) is discussed and it is suggested that relativistic schemes more sophisticated than the simple quasirelativistic approach are needed for 5d-elements. (C) 1998 Elsevier Science B.V. All rights reserved.