Valency assignment, high temperature systems

It was proposed that species B is a five-coordinate distorted trigonal bipyramidal, and C is a distorted square pyramidal species. Both of them are either high- or intermediate-spin Fe" +, although valence assignment in these systems with significant covalent character is rather dubious. The two species are separated by a stereochemical barrier, and there is a dynamic equilibrium between them that shifts toward C at higher temperatures. Such stereochemical nonrigidity in five-coordinate systems has been well known [5]. 

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