A Step beyond the Feltham–Enemark Notation: Spectroscopic and Correlated ab Initio Computational Support for an Antiferromagnetically Coupled M(II)–(NO)⁻ Description of Tp*M(NO) (M = Co, Ni)

Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}⁹ species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me₂-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co–NO bonding interaction as a Co(II) (S_Co = ³/₂) metal center, antiferromagnetically coupled to a triplet NO^– anion (S_NO = 1), an interpretation of the electronic structure that was validated by ab initio multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant g-anisotropy in the S = ¹/₂ ground state, but the linear-response DFT performed poorly at calculating the g-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin–orbit coupling were required for obtaining accurate modeling of the molecular g-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (S_Ni = 1) metal center antiferromagnetically coupled to an S_NO = 1 NO^–. The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO⁰ formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and ab initio computational methods for their ability to predict various spectroscopic and molecular features.