Anti-ferromagnetic alignment and ligand polarization in a spin crossover molecular salt: Theory vs. experiment

For the dinuclear spin crossover [Fe(qsal) 2 ][Ni(dmit) 2 ] (where qsal N(8quinolyl) salicylaldimine and 1,3-dithiol-2-thione-4,5-dithiolato) complex, elemental X-ray magnetic circular dichroism (XMCD) measurements at the Fe and Ni X-ray absorption edges indicate anti-ferromagnetic alignment of the Fe and Ni. This anti-ferromagnetic alignment is consistent with the ab initio calculations performed using the DFT+ U method. The DFT+ U calculations indicate that while the moments residing in the Fe cation are localized on the metal, there is a significant delocalization of the spin density on the Ni(dmit) 2 moiety, with considerable magnetic moment associated with the ligand. This placement of some moment on the dmit ligand explains the XMCD signal at photon energies well above the metal absorption edge. This dmit ligand moment is consistent with the large inverse photoemission signal that is most sensitive to the surface layer dmit, the known surface termination of [Fe(qsal) 2 ][Ni(dmit) 2 ]. The organic ligand clearly can mediate antiferromagnetic coupling between metal centers with significant separation.