Valency assignment, high temperature

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]. 

Doublet 1 may be assigned unambiguous to P4 symmetry, since the hyperfme parameters follow the expected behaviour for Fe in a relatively undistorted tetrahedral site. The high value of quadrupole splitting indicates a high valence contribution, and displays the expected temperature variation for tetrahedral environments bb 1968). The characteristic Mossbauer temperature 0m is in the range expected for Fe (De Grave and Van Alboom 1991). 

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