Prediction of 195Pt NMR Chemical Shifts by Density Functional Theory Computations: The Importance of Magnetic Coupling and Relativistic Effects in Explaining Trends

Density functional theory with relativistic corrections has been used to calculate the 195 Pt chemical shifts for a series of Pt(II) complexes. Good agreement with experimental values is observed with two different relativistic correction methods. Deconvolution of the parameters leading to the overall shielding of the platinum nucleus shows that both the paramagnetic and the spin−orbit shielding terms contribute substantially. Detailed transition analysis demonstrates that the most important contributions to the paramagnetic shielding for PtX 4 2- anions and cis - and trans -PtX 2 (NH 3 ) 2 compounds come from the Pt d xy −X lone pair π → Pt d x 2 - y 2 −X σ* and Pt d xy −X lone pair π* → Pt d x 2 - y 2 −X σ* transitions, in accord with qualitative predictions. For cis - and trans -PtX 2 L 2 complexes (L = PMe 3, AsMe 3, SMe 2 ), the Pt d xy −X lone pair π → Pt d x 2 - y 2 −X σ* transition is most important, but the Pt d xy −X lone pair π* → Pt d x 2 - y 2 −X σ* transition is much less so. This is readily understood through recognition of the importance of the magnetic coupling term to the paramagnetic shielding. The trend that chemical shifts vary as I - < Br - < Cl - arises from the magnetic coupling term and the spin−orbit contribution; it runs counter to the trend predicted by the energy gaps between the orbitals involved in the important transitions.