Mixed Spin-State [HS-LS] Pairs in a Dinuclear Spin-Transition Complex: Confirmation by Variable-Temperature57Fe Mössbauer Spectroscopy

Exquisite sensitivity of Mossbauer spectroscopy for tiny local molecular distortion is demonstrated in [FeII2(pmat)2](BF4)4?DMF: high-spin (HS) iron(II) in [HS-HS] and in [LS-HS] (low-spin–high-spin) pairs is clearly distinguished (see picture) for the first time without the need to apply a magnetic field. This dinuclear complex clearly shows that spin crossover via a [LS-HS] species is promoted by the use of a highly constrained bridging ligand (the bis-terdentate pmat).