Mössbauer effect study of the electronic ground state of iron(II) in [57FexM1−x(bipy)3](ClO4)2 (M = Mn, Ni, Zn) and [57FexM1−x(phen)3](ClO4)2 (M = Ni, Zn) at very lowiron concentrations

Abstract Earlier work in our laboratory on the effect of metal dilution on the 5 T 2 (O h ⇌ 1 A 1 (O h ) equilibrium in polycrystalline spin crossover systems of iron(II) has shown that the relative stability of the high spin state, 5 T 2 (O h ), at a given temperature, increases markedly with decreasing iron concentration. These results have initiated the present work. Using 57 Fe Mossbauer spectroscopy, we have investigated the electronic ground state of iron(II) in the highly diluted solid solutions [Fe x M 1−x L 3 ] with L = bipy, M = Mn, Ni, Zn, x ≲ 0.005 and L = phen, M = Ni, Zn, x ≲ 0.005. Although the critical field potential (V c ) of the pure iron complexes, which are known to be low spin, is reported to be not far from the crossover point V c = P ( P , the mean spin pairing energy), the effect of metal dilution does not reduce the difference ¦ V c - P ¦ sufficiently as to thermally populate the 5 T 2 (O h ) state at room temperature to any noticeable extent. ¦ V c - P ¦ has been estimated to be ≳ 1200 cm −1 . The quadrupole splitting and the resonance line width are nearly the same in the pure compounds and the solid solutions. The isomer shift, however, is found to increase slightly in the order M = Fe ++ host ions.