Understanding the Electronic Factors Responsible for Ligand Spin–Orbit NMR Shielding in Transition-Metal Complexes

The significant role of relativistic effects in altering the NMR chemical shifts of light nuclei in heavy-element compounds has been recognized for a long time; however, full understanding of this phenomenon in relation to the electronic structure has not been achieved. In this study, the recently observed qualitative differences between the platinum and gold compounds in the magnitude and the sign of spin–orbit-induced (SO) nuclear magnetic shielding at the vicinal light atom (<sup>13</sup>C, <sup>15</sup>N), σ<sup>SO</sup>(LA), are explained by the contractions of 6s and 6p atomic orbitals in Au complexes, originating in the larger Au nuclear charge and stronger scalar relativistic effects in gold complexes. This leads to the chemical activation of metal 6s and 6p atomic orbitals in Au complexes and their larger participation in bonding with the ligand, which modulates the propagation of metal-induced SO effects on the NMR signal of the LA via the Spin–Orbit/Fermi Contact (SO/FC) mechanism. The magnitude of the σ<sup>SO</sup>(LA) in these square-planar complexes can be understood on the basis of a balance between various metal-based 5d → 5d* and 6p → 6p* orbital magnetic couplings. The large and positive σ<sup>SO</sup>(LA) in platinum complexes is dominated by the shielding platinum-based 5d → 5d* magnetic couplings, whereas small or negative σ<sup>SO</sup>(LA) in gold complexes is related to the deshielding contribution of the gold-based 6p → 6p* magnetic couplings. Further, it is demonstrated that σ<sup>SO</sup>(LA) correlates quantitatively with the extent of M–LA electron sharing that is the covalence of the M–LA bond (characterized by the QTAIM delocalization index, DI). The present findings will contribute to further understanding of the origin and propagation of the relativistic effects influencing the experimental NMR parameters in heavy-element systems.