Orbital-Dependent Magnetic Exchange in Octahedral Cobalt(II): An Ab Initio Perspective

Here, we investigate the magnetic exchange coupling and magnetic anisotropy of a Co(II)-Cu(II) complex using ab initio multiconfigurational calculations. This is one of the simplest cases of magnetic exchange coupling involving an orbitally degenerate metal center: a pseudo-octahedral 3d7 Co(II) ion and a spin-only 3d9 Cu(II) ion. Electronic structure calculations were performed using CASSCF and CASPT2/NEVPT2 methods, and the resulting electronic states were further analyzed via model Hamiltonians to extract physically meaningful parameters, including crystal field splitting, spin-orbit coupling, and exchange coupling. The CASPT2/NEVPT2 calculations improved the agreement with experiment compared to that of CASSCF by correcting the overestimation of ferromagnetic coupling. Our calculations highlight that spin-orbit exchange coupling (i.e., spin-other-orbit coupling) is just as important as spin-spin exchange coupling for an accurate description of exchange coupling.