The observation of single-molecule magnetism in transition-metal complexes relies on the phenomenon of zero-field splitting (ZFS), which arises from the interplay of spin−orbit coupling (SOC) with ligand-field-induced symmetry lowering. Previous studies have demonstrated that the magnitude of ZFS in complexes with 3d metal ions is sometimes enhanced through coordination with heavy halide ligands (Br and I) that possess large free-atom SOC constants. In this study, we systematically probe this "heavy-atom effect" in high-spin cobalt(II)−halide complexes supported by substituted hydrotris(pyrazol-1-yl)borate ligands (Tp tBu,Me and Tp Ph,Me ). Two series of complexes were prepared: [Co II X(Tp tBu,Me )] (1-X; X = F, Cl, Br, and I) and [Co II X(Tp Ph,Me )(Hpz Ph,Me )] (2-X; X = Cl, Br, and I), where Hpz Ph,Me is a monodentate pyrazole ligand. Examination with dc magnetometry, high-frequency and -field electron paramagnetic resonance, and far-infrared magnetic spectroscopy yielded axial (D) and rhombic (E) ZFS parameters for each complex. With the exception of 1-F, complexes in the four-coordinate 1-X series exhibit positive D-values between 10 and 13 cm −1 , with no dependence on halide size. The five-coordinate 2-X series exhibit large and negative D-values between −60 and −90 cm −1 . Interpretation of the magnetic parameters with the aid of ligand-field theory and ab initio calculations elucidated the roles of molecular geometry, ligand-field effects, and metal−ligand covalency in controlling the magnitude of ZFS in cobalt−halide complexes.

A series of mononuclear Co(II) complexes with noninnocent (redox-active) ligands are prepared that exhibit metal− ligand cooperativity during the reversible binding of O 2 . The complexes have the general formula, [Co II (L S,N )(Tp R2 )] (R = Me, Ph), where L S,N is a bidentate o-aminothiophenolate and Tp R2 is a hydrotris(pyrazol-1-yl)borate scorpionate with R-substituents at the 3and 5-positions. Exposure to O 2 at room temperature results in oneelectron oxidation and deprotonation of L S,N . The oxidized derivatives possess substantial "singlet diradical" character arising from antiferromagnetic coupling between an iminothiosemiquinonate (ITSQ •− ) ligand radical and a low-spin Co(II) ion. The [Co II (Tp Me2 )-( X2 ITSQ)] complexes, where X = H or tBu, coordinate O 2 reversibly at reduced temperatures to provide Co/O 2 adducts. The O 2 binding reactions closely resemble those previously reported by our group (Kumar et al., J. Am. Chem. Soc. 2019,141, 10984−10987) for the related complexes [Co II (Tp Me2 )( tBu2 SQ)] and [Co II (Tp Me2 )( tBu2 ISQ)], where tBu2 (I)SQ represents 4,6-di-tert-butyl-(2-imino)semiquinonate radicals. In each case, the oxygenation reaction proceeds via the addition of O 2 to both the cobalt ion and the ligand radical, generating metallocyclic cobalt(III)-alkylperoxo structures. Thermodynamic measurements elucidate the relationship between O 2 affinity and redox potentials of the (imino)(thio)semiquinonate radicals, as well as energetic differences between these reactions and conventional metal-based oxygenations. The results highlight the utility and versatility of noninnocent ligands in the design of O 2 -absorbing compounds.