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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)

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journal contribution
posted on 22.02.2016 by Neil C. Tomson, Mark R. Crimmin, Taras Petrenko, Lauren E. Rosebrugh, Stephen Sproules, W. Christopher Boyd, Robert G. Bergman, Serena DeBeer, F. Dean Toste, Karl Wieghardt
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) (SCo = 3/2) metal center, antiferromagnetically coupled to a triplet NO anion (SNO = 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) (SNi = 1) metal center antiferromagnetically coupled to an SNO = 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.