Mixed Spin‐State [HS‐LS] Pairs in a Dinuclear Spin‐Transition Complex: Confirmation by Variable‐Temperature 57 Fe Mössbauer Spectroscopy

Exquisite sensitivity of Mössbauer 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).