Origin of Ferromagnetism in the Copper(II) Triangle NMe₄[Cu₃(μ₃-F)(TFA)₆(Py)₃]

Magnetic interactions in transition metal compounds are defined by the Goodenough-Kanamori rules. Experimental magnetic susceptibility and high-field electron paramagnetic resonance experiments on the triangular molecular magnet NMe₄[Cu₃(μ₃-F)(TFA)₆(Py)₃] [1] reveal dominant ferromagnetic spin-spin exchange as well as the importance of anisotropy in the system. Based on the Goodenough-Kanamori rules one would expect anti-ferromagnetic exchange interactions. Density functional theory and ab initio full multiplet ligand-field theory calculations confirm the conclusions drawn from the Goodenough-Kanamori rules, contradicting experimental observations. Only once correlations on the central ligand fluorine ion are considered within the theory we predict the magnetic interactions in agreement with experiment. Our results show the importance of correlations on the ligands for the understanding of magnetic interactions. These results should be generally valid and we discuss when correlations on the ligands change the magnetic interactions in transition metal compounds like Fe₃O₄ or doped H_Tc cuprate superconductors.

[1] Tereshchenko, D.S., Buzoverov, M.E., Glazunova, Russ. J. Inorg. Chem. (2023). https://doi.org/10.1134/S0036023623601666