Atomic Contributions from Spin-Orbit Coupling to 29Si NMR Chemical Shifts in Metallasilatrane Complexes

New members of a novel class of metallasilatrane complexes [X-Si-(mu-mt)(4)-M-Y], with M=Ni, Pd, Pt, X=F, Cl, Y=Cl, Br, I, and mt=2-mercapto-1-methylimidazolide, have been synthesized and characterized structurally by X-ray diffraction and by Si-29 solid-state NMR. Spin-orbit (SO) effects on the Si-29 chemical shifts induced by the metal, by the sulfur atoms in the ligand, and by heavy halide ligands Y=Cl, Br, I were investigated with the help of relativistic density functional calculations. Operators used in the calculations were constructed such that SO coupling can selectively be switched off for certain atoms. The unexpectedly large SO effects on the Si-29 shielding in the Ni complex with X=Y=Cl reported recently originate directly from the Ni atom, not from other moderately heavy atoms in the complex. With respect to Pd, SO effects are amplified for Ni owing to its smaller ligand-field splitting, despite the smaller nuclear charge. In the X=Cl, Y=Cl, Br, I series of complexes the Y ligand strongly modulates the Si-29 shift by amplifying or suppressing the metal SO effects. The pronounced delocalization of the partially covalent M <- Y bond plays an important role in modulating the Si-29 shielding. We also demonstrate an influence from the X ligand on the Si-29 SO shielding contributions originating at Y. The NMR spectra for [X-Si-(mu-mt)(4)-M-Y] must be interpreted mainly based on electronic and relativistic effects, rather than structural differences between the complexes. The results highlight the sometimes unintuitive role of SO coupling in NMR spectra of complexes containing heavy atoms.