An unusual example of a rapid spin-equilibrium iron complex

Bis(N-(1-benzoyl-2-propylidene)-2-pyridylmethylaminato)iron(III) hexafluorophosphate (Fe(bzpa)₂PF₆ is shown to be a spin-equilibrium compound between low- (S = $\text{1}\text{2}$) and high- (S = $\text{5}\text{2}$) spin states depending on temperature, and to have the unusual property that the relaxation time of the spin change is shorter than the Mössbauer lifetime (1 × 10^?7s) of iron- 57 (l = $\text{3}\text{2}$) in the solid state.