10.1021/ja206042k.s002 Neil C. Tomson Neil C. Tomson Mark R. Crimmin Mark R. Crimmin Taras Petrenko Taras Petrenko Lauren E. Rosebrugh Lauren E. Rosebrugh Stephen Sproules Stephen Sproules W. Christopher Boyd W. Christopher Boyd Robert G. Bergman Robert G. Bergman Serena DeBeer Serena DeBeer F. Dean Toste F. Dean Toste Karl Wieghardt Karl Wieghardt A Step beyond the Feltham–Enemark Notation: Spectroscopic and Correlated <i>ab Initio</i> Computational Support for an Antiferromagnetically Coupled M(II)–(NO)<sup>−</sup> Description of Tp*M(NO) (M = Co, Ni) American Chemical Society 2016 0 formulation EPR Cu analogues SNO CASSCF Correlated ab Initio Computational Support Ni complexes contrasts ab initio multireference methods DFT Tp diamagnetic Ni analogue ab initio metal center 2016-02-22 13:40:23 Journal contribution https://acs.figshare.com/articles/journal_contribution/A_Step_beyond_the_Feltham_Enemark_Notation_Spectroscopic_and_Correlated_i_ab_Initio_i_Computational_Support_for_an_Antiferromagnetically_Coupled_M_II_NO_sup_sup_Description_of_Tp_M_NO_M_Co_Ni_/2580547 Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}<sup>9</sup> species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me<sub>2</sub>-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) (<i>S</i><sub>Co</sub> = <sup>3</sup>/<sub>2</sub>) metal center, antiferromagnetically coupled to a triplet NO<sup>–</sup> anion (<i>S</i><sub>NO</sub> = 1), an interpretation of the electronic structure that was validated by <i>ab initio</i> multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant <i>g</i>-anisotropy in the <i>S</i> = <sup>1</sup>/<sub>2</sub> ground state, but the linear-response DFT performed poorly at calculating the <i>g</i>-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 <i>g</i>-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) (<i>S</i><sub>Ni</sub> = 1) metal center antiferromagnetically coupled to an <i>S</i><sub>NO</sub> = 1 NO<sup>–</sup>. The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO<sup>0</sup> formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and <i>ab initio</i> computational methods for their ability to predict various spectroscopic and molecular features.