Magnetic Anisotropy in the Homoleptic [CoX4]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.