Prediction of Pt-195 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 ca lculate the Pt-195 chemical shifts for a series of Pt(II) complexes. Good a greement with experimental values is observed with two different relativist ic correction methods. Deconvolution of the parameters leading to the overa ll shielding of the platinum nucleus shows that both the paramagnetic and t he spin-orbit shielding terms contribute substantially. Detailed transition analysis demonstrates that the most important contributions to the paramag netic shielding for PtX42- anions and cis- and trans-PtX2(NH3)(2) compounds come from the Pt d(xy)-X lone pair pi --> Pt d(x2-y2)-X sigma* and Pt d(xy )-X lone pair pi* --> Pt d(x2-y2)-X sigma* transitions, in accord with qual itative predictions. For cis- and trans-PtX2L2 complexes (L = PMe3, AsMe3, SMe2), the Pt d(xy)-X lone pair pi --> Pt d(x2-y2)-X sigma* transition is m ost important, but the Pt d(xy)-X lone pair pi* --> Pt d(x2-y2)-X sigma* tr ansition 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 mag netic coupling term and the spin-orbit contribution; it runs counter to the trend predicted by the energy gaps between the orbitals involved in the im portant transitions.