Ligand Field Determines g-Tensor Anisotropy and Radicaloid Character of (L)RuV=O and (L)RuV=N Catalytic Intermediates

Abstract High-valent metal-oxo and metal-imido species mediate complex chemical transformations, yet their electronic structures remain difficult to characterize experimentally and computationally because of their short-lived nature and pronounced multiconfigurational character. The RuV (S = 1/2) complexes investigated here promote O–O bond formation in water oxidation and enable C(sp3)–H bond amination. Using multiconfigurational CASSCF/NEVPT2 calculations combined with wave function analyses, we demonstrate that g-tensor anisotropy, accessible via EPR measurements, is governed by spin–orbit coupling to low-lying doublet excited states whose energies are determined by the ligand field. Wave function analyses reveal that these transitions originate from the RuV=X (X=O, N) bonding and reflect varying degrees of RuIV-X• oxyl character. Our calculations reproduce the experimentally observed low g-tensor anisotropy of established RuV=O and RuV=N (S = 1/2) species. However, two previously reported highly anisotropic EPR signals are difficult to reconcile with a RuV assignment and can be more readily explained by proposed alternative RuIII species, which have low-lying spin–orbit-coupled excited states giving rise to the high g-tensor anisotropy. Overall results establish a quantitative relationship between the ligand field, which determines the excited-state electronic structure and radicaloid character of RuV=O and RuV=N fragments. These, in addition, manifest themselves in visualized natural orbitals and via spin–orbit selection rules, in the g-tensor anisotropy, elucidating the origins of EPR spectral features of high-valent transition-metal intermediates. Overall, our quantitative analysis establishes low g-tensor anisotropy in the EPR spectrum as a diagnostic spectroscopic signature of a high-valent RuV intermediate, should such a species be generated.