Ferrates isomer shift values

Significantly, isomer shift values of ferrates decreased with increase in oxidation state (OS) of iron in ferrates (Fig. 24.8). The decrease was linear and can be expressed by Eq. (24.1). 

Decomposition oflron(V) in Air The Mossbauer spectra of ferrate(V) during decomposition for 12 days have been studied [21]. An initial spectrum had an isomer shift value of approximately —0.55 mms for ferrate(V), which slowly disappeared in I2 days. The decomposition of ferrate(V) gave an intense line of iron(VI) (S= —0.90mms ) and unresolved wide line, which may be lines of +3 and +4 oxidation states of iron. 

Fig. 4.3 Ranges of isomer shifts observed for Fe compounds relative to metallic iron at room temperature (adapted from [24] and complemented with recent data). The high values above 1.4-2 mm s were obtained from Co emission experiments with insulators like NaCl, MgO or Ti02 [25-28], which yielded complex multi-component spectra. However, the assignment of subspectra for Fe(I) to Fe(III) in different spin states has never been confirmed by applied-field measurements, or other means. More recent examples of structurally characterized molecular Fe (I)-diketiminate and tris(phosphino)borate complexes with three-coordinate iron show values around 0.45-0.57 mm s [29-31]. The usual low-spin state for Fe(IV) with 3d configuration is 5 = 1 for quasi-octahedral or tetrahedral coordination. The low-low-spin state with S = 0 is found for distorted trigonal-prismatic sites with three strong ligands [30, 32]. Occurs only in ferrates. There is only one example of a molecular iron(VI) complex it is six-coordinate and has spin S = 0 [33]...

Equation (24.1) was used to estimate isomer shifts of ferrate(VII) and ferrate(VIII) as — 1. 18 and — 1.40 mm s , relative to a-Fe, respectively [102]. More recently, density function theory calculations were performed on ferrates ranging from +4 to +8 oxidation states of iron [102]. Figure 24.9 depicts relationship of isomer shift with Fe O bond length and electron density on the iron nucleus An increase in Fe—O bond distance with decrease in oxidation states of iron from ferrate(VIII) to ferrate(IV) was determined. A positive linearity between isomer shift and bond distance of ferrate was found (Fig. 24.9). Conversely, values of isomer shifts showed a negative relationship with po (Fig- 24.9). An increase in the Po with increase of the oxidation state of iron was expected. The relationship of Eq. (24.1) was used to understand the mechanism of decomposition and formation reactions of solid ferrates under different conditions, which are described below. 

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