Isomer shift compounds

The spin state of the compounds XFe(R2dtc)2 is 3/2 (64). Mossbauer spectra of ClFe(Et2 tc)2 in solution are almost identical with the spectrum of the six-coordinated Fe(Et2magnetic susceptibility and in the isomer shift and quadrupole splitting parameters suggests a geometrical correspondency in solution, which can be attained by the binding of a solvent molecule to the sixth coordination site of the ClFe(Et2[Pg.98]

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

A calibration of the popular B3LYP and BP86 density functionals for the prediction of Fe isomer shifts from DFT calculations [16], using a large number of complexes with a wide range of iron oxidation states and a span of about 2 mm s for the isomer shifts, yielded a value for the calibration constant a = —0.3666 mm s a.u. (see Chap. 5). Note the negative sign, which indicates that a positive isomer shift of a certain compound relative to a reference material reveals a lower electron density at the nuclei in that compound as compared to nuclei in the reference material. 

High-spin iron compounds the lower the oxidation state the more positive is the isomer shift. Note that the allocation of high-spin iron(ll) is unique for -values >1 mm s. ... 

Low-spin iron compounds isomer shifts are rather similar. It is, for example, not possible to distinguish between low-spin iron(ll) and low-spin iron(lll) configurations from (5-values alone. 

Low-spin compounds exhibit lower isomer shifts than high-spin compounds, whereas the isomer shifts of intermediate-spin compounds often resemble those of the corresponding low-spin compounds. 

In addition to the variation in electronic configuration, the geometric details of the coordination sphere and the properties of iron-ligand bonds (different a- or 71-donor strength) also influence the isomer shift as observed for a series of compounds ... 

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

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]...

Mossbauer isomer shifts of iron-containing compounds are traditionally explained... 

Given the success of DFT in the calculation of the isomer shift, it seems appropriate to return to the issue of interpretation which factors are controlling the qualitative behavior of the isomer shift in iron compounds Traditionally, one assumes that there is a correlation of the isomer shift and the charge at the iron center as is suggested from the well-known sensitivity of the isomer shift with respect to the oxidation state. However, things turn out to be more subtle than what is perhaps commonly perceived. 

Isomer shifts have been measured in a variety of nickel compounds. In most cases, however, the information concerning chemical bond properties was not very impressive. The reason is that the second-order Doppler (SOD) shift is, in many systems, of comparable magnimde as the real chemical isomer shift, which causes... 

The first Ni Mossbauer spectrum of nickel in a bioinorganic compound with determinable EFG and isomer shift was reported for a nickel complex compound with planar [NiSJ core and considered as a model compound for hydrogenase. This Mossbauer spectrum from the formal Ni compound is presented in Fig. 7.16. The observed quadrupolar interaction can be understood in terms of ligand field theory. In this approach, the b g and levels (d y2 and d ) are not occupied which is expected to cause a large negative EFG contribution [32]. 

Fig. 7.30 Measured Ru isomer shifts in ruthenium compounds with different oxidation states of Ru (from [113] and complemented by values obtained at 4.2 K for Ru(V) oxides of the form Ba3Ru2M09 (M = Mg Ca, Sr Co, Ni, Zn and Cd) from [114])...

A similar situation with changing oxidation state of Ru within a series of compounds was observed for the ordered perovskites BaLaMRuOe (M = Mg, Fe, Co, Ni, Zn) [115]. The measured isomer shifts in the range +0.06 to +0.13 mm s ... 

It is further noteworthy that K4[Ru(CN)6] 3H2O shows an isomer shift close to the shifts of Ru(lV) compounds, from which it is inferred that practically two t2g electrons have been delocalized from the Ru(II) cluster to the strong 7t-bonding CN ligands [123]. 

In another investigation of bond properties in ruthenium compounds, it was found [123] that the isomer shift increases in the series... 

Fig. 7.31 Partial ligand-field strength for the ligand L in correlation with the isomer shift 5 (relative to Ru metal at 4.2 K) of the nitrosylruthenium (II) compounds [RuL5(NO)]" (L = Br, Cr, NCS, NH3, CN ) (from [124])...

A correlation of isomer shift, electronic configuration, and calculated -electron densities for a number of ruthenium complexes in analogy to the Walker-Wertheim-Jaccarino diagram for iron compounds has been reported by Clausen et al. [ 127]. Also useful is the correlation between isomer shift and electronegativity as communicated by Clausen et al. [128] for ruthenium trihalides where the isomer shift appears to increase with increasing Mulliken electronegativity. 

Various Ru-oxides, YBa2Cu307, c (I), Ba Ru2/3Gdi/303 (II) as well as Ru-doped a-Fe203 (III), to probe the local chemical structure around the Ru atoms. Compound (I) has interesting properties with x < 0.2 it is a superconductor and with x 1 a semiconductor. Ru oxidation state and coordination are discussed on the basis of measured isomer shifts and quadrupole splittings Ru(IV) ions exclusively occupy Cu-1 sites which form one-dimensional chains... 

The isomer shifts in hafnium Mossbauer isotopes usually are of the order of some percent of the line width. Boolchand et al. [168] observed a relatively large isomer shift of -1-0.19 0.06 mm s between cyclopentadienyl hafnium dichloride (Hf(Cp)2Cl2) and Hf metal. From a comparison with Os(Cp)2 and Os-metal, a value of 5 r ) ( Hf) = —0.37 10 fm has been derived, which implies a shrinking of the nuclear radius in the excited 2 state. Figure 7.37 shows some typical spectra for Hf in various hafnium compounds (from [168]). 

Fig. 7.43 Graphical representation of isomer shifts of the 6.2 keV y-transition in Ta for tantalum compounds and for dilute impurities of Ta in transition metal hosts (from [186])...

---