Magnetic Anisotropy in the Homoleptic [CoX ₄ ] ²– (X = Cl, Br, I) Series: Spectroscopic Determination and Ligand Field Studies

Four-coordinate transition metal complexes with unpaired electrons (S ≥ 1) typically exhibit structures deviated from perfect Td geometry, leading to magnetic anisotropy. (NEt4)2[CoX4] (X = Cl, Co–Cl; Br, Co–Br; I, Co–I) with pseudotetrahedral structures are an ideal series to explore how deviation from perfect Td geometry is reflected in magnetic anisotropy. This work presents comprehensive studies of Co-X, including single-crystal X-ray diffraction of Co–Cl and Co–I, measurements of DC and AC magnetic susceptibilities, inelastic neutron scattering (INS), far-infrared magneto-spectroscopy (FIRMS), and high-frequency and -field electron paramagnetic resonance (HFEPR). Both [CoCl4]2– and [CoBr4]2– ions have crystallographically imposed D2d symmetry, while the [CoI4]2– ion adopts slightly distorted tetrahedral geometry approximating D2d symmetry. Magnetic anisotropy increases from Co–Cl to Co–I. Also, only Co–Cl and Co–Br show field-induced SMM behaviors. FIRMS of Co–I reveals spin-phonon couplings, suggesting that these couplings may lead to fast magnetic relaxation and the lack of SMM behavior. Ligand field theory calculations indicate that an increase in spin-orbit couplings (SOC) from Co–Cl to Co–Br and to Co–I leads to increased magnetic anisotropy. These compounds provide insight into how crystal fields, crystallographic symmetries, and SOC affect magnetic anisotropy and spin relaxation in a well-defined series of homoleptic complexes.