Spin state transition in iron(II): a review on bis-[(2,6-bis(pyrazol-3-yl)pyridine]iron(II) complex

Iron(II) has been known to exist as high-spin ( 5 T 2 ) and low-spin ( 1 A 1 ) states. The first and the second states are due to the weak and strong ligand fields of the corresponding complexes, respectively. The magnetic moments of the two states are then immediately distinguished, being about 4.9-5.4 BM due to four unpaired electrons, but 0-0.6 BM due to no unpaired electrons in the corresponding electronic configuration of 3 d 6 system for the former and the latter, respectively. It could be reasonably understood that in the "middle" ligand field strength the magnetic moment may no longer reflect neither fully high-spin nor fully low-spin but to be in between the two, and thus in this situation the concentration of the two fractions may be induced by temperature or pressure. Thus, the term spin-state transition or spin crossover in iron(II) or spin equilibium of 1 A 1 ⇋ 5 T 2 are then introduced. Indeed, many complexes of [FeN6] 2+ were found to be temperature dependence associated with the spin-state transition. In this paper, the iron(II) complexes of six N-coordinating agent of five-six membered rings and its character of magnetism, Mössbauer and UV-Vis electronic spectra as well as bond lengths are discussed.