Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes

The electronic structure and ground spin states, S , observed for mixed-valent iron–sulfur dimers (Fe II -Fe III ) are typically determined by the Heisenberg exchange interaction, J , that couples the magnetic interaction of the two metal centres either ferromagnetically ( J > 0, S = 9/2) or antiferromagnetically ( J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B , which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mössbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe 2 Q 2 ] + (Q = S 2– , Se 2– , Te 2– ), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling.