(Tetrakis(2-pyridylmethyl)ethylenediamine)iron(II) Perchlorate, the First Rapidly Interconverting Ferrous Spin-Crossover Complex.

The preparation and characterization of the first Fe II spin-crossover complex that interconverts between high- and low-spin states at a rate faster than the 57 Fe Mossbauer time scale is reported. [Fe(tpen)](ClO 4 ) 2 .2/3H 2 O crystallizes in the monoclinic space group C2/c.