Magneto-structural relationships for a mononuclear Co(II) complex with large zero-field splitting

A mononuclear cobalt(II) complex, [Co(ac)2(H2O)2(MeIm)2], with heteroleptic coordination sphere possessing the {CoO2O′2N2} chromophore has been prepared and structurally characterized. The magnetic data down to 2 K show an enhanced magnetic anisotropy manifesting itself in a large zero-field splitting (ZFS) parameter. As a consequence, the magnetization deviates substantially from the Brillouin-function behavior. A fit to the zero-field splitting model gave the following set of magnetic parameters: D/hc = +95 cm−1, gx = 2.530, zj/hc = −0.078, χTIP = 16.7 × 10−9 m3 mol−1, (gz = 2.0). The Griffith–Figgis model and the Generalized Crystal-Field model lie beyond the spin-Hamiltonian formalism; they gave analogous, although not identical ZFS parameters: D/hc = 109 cm−1, and D/hc = 77 cm−1, respectively. The absorption spectrum taken in the FAR-IR region exhibits manifold absorption peaks referring to the transitions among the crystal-field multiplets of the parent 4A2g + 4Eg terms (D4h), originating in a crystal-field splitting of the octahedral 4T1g ground term.