Quadratic Spin–Orbit Mechanism of the Electronic g‑Tensor

Understanding
how the electronic g-tensor is linked to the electronic
structure is desirable for the correct interpretation of electron
paramagnetic resonance spectra. For heavy-element compounds with large
spin–orbit (SO) effects, this is still not completely clear.
We report our investigation of quadratic SO contributions to the g-shift
in heavy transition metal complexes. We implemented third-order perturbation
theory in order to analyze the contributions arising from frontier
molecular spin orbitals (MSOs). We show that the dominant quadratic
SO termspin-Zeeman (SO2/SZ)generally makes
a negative contribution to the g-shift, irrespective of the particular
electronic configuration or molecular symmetry. We further analyze
how the SO2/SZ contribution adds to or subtracts from the
linear orbital-Zeeman (SO/OZ) contribution to the individual principal
components of the g-tensor. Our study suggests that the SO2/SZ mechanism decreases the anisotropy of the g-tensor in early transition
metal complexes and increases it in late transition metal complexes.
Finally, we apply MSO analysis to the investigation of g-tensor trends
in a set of closely related Ir and Rh pincer complexes and evaluate
the influence of different chemical factors (the nuclear charge of
the central atom and the terminal ligand) on the magnitudes of the
g-shifts. We expect our conclusions to aid the understanding of spectra
in magnetic resonance investigations of heavy transition metal compounds.