Five-coordinate ferric compounds

Another typical example for anisotropic covalency is found in five-coordinate ferric compounds with intermediate spin S = 3/2 (also discussed in Sect. 8.2). Crystal field theory predicts a vanishing valence contribution to the EFG, whereas large quadrupole splittings up to more than 4 mm s are experimentally found. 

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an... 

Table 8.1 Mossbauer parameters of five- and six-coordinate ferric compounds with 5pe = 3/2...

In normal Hb, the v(Fe—N) of the proximal histidine (F8) is near 220 cm-1 (6). In mutant Hb such as Hb M Iwate and Hb M Boston, F8 histidine and E7 histidine are replaced by tyrosine residues, respectively. In five-coordinate ferric a subunits of these compounds, the v(Fe—O- (phenolate)) bands are observed at 589 and 603 cm-1, respectively (7). 

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