An Unprecedented Square Planar Fe(III) Complex Exhibiting Spin Crossover Between the Spin‐Admixed States

The presence of strong spin‐orbit coupling, which is comparable to the crystal field in certain ferric‐porphyrin/phthalocyanine derivatives, results in a spin‐admixed state between the high‐spin (HS; S = 5/2) and intermediate‐spin (IS; S = 3/2) states. However, there are no literature precedents for spin admixing between the IS ( S = 3/2) and low‐spin (LS; S = 1/2) states, which are purely governed by axial ligation. It is also true that systems showing spin‐admixed behavior do not exhibit thermal equilibrium between the spin states, that is, spin crossover (SCO). In this article, we report a unique and rare square planar ferric ion complex, [Na(DME) 3 ][Fe(L1 2− ) 2 ] ( 1 ), which exhibits two different physical phenomena: SCO is observed between the spin‐admixed states, a phenomenon unprecedented in this context. Field‐ and temperature‐dependent Mössbauer spectra, variable temperature X‐band EPR spectra, multifrequency EPR (MF‐EPR), and temperature‐dependent magnetic susceptibility measurements reveal that the spin‐admixed state has contributions from IS ( S = 3/2) and LS ( S = 1/2), unlike other reports in the literature. The detailed electronic structure of 1 , along with the rationale for quantum spin admixing and SCO, is well supported by theoretical calculations.