Oxidation state isomers

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

Such synthons with o-phenylenediamines afford 2(lF/)-quinoxalmones a single product or two isomers will be formed according to the symmetry of the substrate. Related synthons at a lower oxidation state produce dUiydroquinoxahnones. The following examples illustrate typical results. 

The isomer shift, d, arises from the Coulomb interaction between the positively charged nucleus and the negatively charged s-electrons, and is thus a measure for the s-electron density at the nucleus, yielding useful information on the oxidation state of the iron in the absorber. An example of a single line spectrum is fee iron, as in stainless steel or in many alloys with noble metals. 

A further dramatic comparison of the comparative reactivities of chromium alkyls in diverse oxidation states was furnished by another set of benzyl complexes. [7] Shown below, these three compound are isomers, yet they range in oxidation state from Cr to Cr °. Of the three, only the mixed-valent complex Cp G0i-ti n3-Bz)Cr(Bz)Cp, containing a Bivalent chromium bound to an T)3-benzyl and a ni-benzyl ligand, catalyzed the polymerization of ethylene. 

In Table 4 typical values are given for the isomer shift (IS) and the quadrupole splitting (QS) of dithiocarbamato complexes with iron in various formal oxidation states. 

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. 

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

A typical example of a correlation diagram for Fe is given in Fig. 4.3. It summarizes the isomer shifts for a great variety of iron complexes with oxidation states (1) to (VI) in the order of the respective high-spin, intermediate-spin, and low-spin configurations. The plot of the corresponding values marked by grey, hatched and open bars demonstrates three major trends ... 

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. ... 

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 shift changes are sufficiently large for Ru atoms in different systems to distinguish between different oxidation states and different bond properties ... 

One of the interesting features in mthenium chemistry is the large variety of oxidation states (0 to - -8) of ruthenium, which may well be distinguished by isomer shift measurements. As an example. Fig. 7.29 shows a few representative single-line spectra [113] reflecting the significant isomer shift changes for different... 

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 ... 

Isomer shift and quadmpole splitting of salts, [Ru(C5H5)X] Y (X = Cl, Br Y = PFg and X = I, Y = I3) are larger compared to those of ruthenocene. This indicates direct chemical bonding between Ru and Cl, Br and I and that the Ru ion in each salt is in an oxidation state higher than Ru(II) in ruthenocene... 

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... 

Fig. 7.59 Ir (73 keV) isomer shifts as a function of oxidation state of iridium (from [268])...

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