posted on 2015-04-14, 00:00authored byJan Vícha, Cina Foroutan-Nejad, Tomasz Pawlak, Markéta L. Munzarová, Michal Straka, Radek Marek
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 (13C, 15N), σSO(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 σSO(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 σSO(LA) in platinum complexes is dominated by the shielding
platinum-based 5d → 5d* magnetic couplings, whereas small or
negative σSO(LA) in gold complexes is related to
the deshielding contribution of the gold-based 6p → 6p* magnetic
couplings. Further, it is demonstrated that σSO(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.