Magnetic coupling between Fe(NO) spin probe ligands through diamagnetic Ni^II, Pd^II and Pt^II tetrathiolate bridges

Reaction of the nitrosylated-iron metallodithiolate ligand, paramagnetic (NO)Fe(N₂S₂), with [M(CH₃CN)][BF₄]₂ salts (M = Ni^II, Pd^II, and Pt^II; n = 4 or 6) affords di-radical tri-metallic complexes in a stairstep type arrangement ([FeMFe]²⁺, M = Ni, Pd, and Pt), with the central group 10 metal held in a MS₄ square plane. These isostructural compounds have nearly identical ν(NO) stretching values, isomer shifts, and electrochemical properties, but vary in their magnetic properties. Despite the intramolecular Fe⋯Fe distances of ca. 6 Å, antiferromagnetic coupling is observed between {Fe(NO)}⁷ units as established by magnetic susceptibility, EPR, and DFT studies. The superexchange interaction through the thiolate sulfur and central metal atoms is on the order of Ni^II < Pd^II ≪ Pt^II with exchange coupling constants (J) of -3, -23, and -124 cm^-1, consistent with increased covalency of the M-S bonds (3d < 4d < 5d). This trend is reproduced by DFT calculations with molecular orbital analysis providing insight into the origin of the enhancement in the exchange interaction. Specifically, the magnitude of the exchange interaction correlates surprisingly well with the energy difference between the HOMO and HOMO-1 orbitals of the triplet states, which is reflected in the central metal's contribution to these orbitals. These results demonstrate the ability of sulfur-dense metallodithiolate ligands to engender strong magnetic communication by virtue of their enhanced covalency and polarizability.