Study of mixed-valence dinuclear iron(II, III) complexes [Fe-2(bpmp)L-2](BF4)(2): Asymmetrically delocalized valence states

We report the temperature dependence of Mossbauer spectra of a mixed-valenc e dinuclear Fe(II) Fe(III) complex, [Fe-2(bpmp)(Ph(CH2)(2)COO-)(2)] (BF4)(2 ) 1, where Hbpmp represents 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-meth ylphenol. Two quadrupole doublets observed at 78 K approach each other as t he temperature is increased. The two high-energy components of a doublet du e to Fe(II) and of that due to Fe(III) become broad in the temperature rang e from 78 to 230 K and then become narrow above 260 K. The temperature depe ndence of the spectra was interpreted by assuming of intramolecular electro n exchanges between two energetically inequivalent vibronic states Fe-A(II) Fe-B(III) and Fe-A(III)Fe-B(II) where one of them is slightly more stable t han the other.