Redox potentials chromium complexes

Thermal deamination of tris(ethylenediamine)chromium(III) complexes is a standard preparative method for cis- and trans-diacidobis(ethylenediamine) complexes421,422 and the thermal behaviour of the starting materials has been related to their crystal structures.423 The cyano complex cis-[Cr(CN)2(en)2]C104 in DMSO undergoes stepwise reduction III— 11 — I at the DME. The standard redox potential for the Cr /Cr11 couple is -1.586 V (versus SCE). 

Cyclic voltammetry and controlled-potential electrolysis are the techniques that have been used to investigate the electrochemistry of oxo-chromium and oxo-molybdenum corrolates. The data have been related to those obtained for similar porphyrin complexes. Redox potentials are reported in Table 17. 

Data on the redox potentials of germylenes, stannylenes, plumbylenes and their complexes are scarce. In fact, only the electrochemistry of dihalogermylenes, dihalostan-nylenes and their complexes with Lewis bases338 as well as with chromium, molybdenum and tungsten pentacarbonyles339 has been studied. 

It can be seen that, on complexation, either the ferrocene or the chromium oxidations become slightly more difficult, but the difference in redox potential decreases with increasing number of SiMe2 units. This suggests that they tend to prevent interactions between the two metal centers. 

Like the corresponding chromium complex shown in Scheme 7-3, the molybdenum-carbene complex (j/ -C5H4Me)Fe jy -C5H3[l-(OEt)C=Mo(CO)5](2-Me) undergoes a single one-electron oxidation in DME solution. The redox potential ( ° = -I- 0.73 V) is nearly coincident with that of the chromium complex (see Table 7-4) [36]. 

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

The reduction potentials for the clathrochelate [CoDm3(dienCo)2] complex fall in the range observed for a number of Co + 2+ couple redox potentials. For the chromium-capped [CoDm3(dienCr)2] complex, they are much more negative and also fall in the range for CJ.3+/2+ reduction. These results show that the reduction in both Co -Coiii-Co and CF -Co -Cr complexes is caused by redox activity of the capping metal ions rather than that of the... 

Two studies of the quenching of excited chromium(iii) complexes by Fe + have appeared. Rates correlate with redox potentials of the Cr ii complex, and this is the main, though not the only reason for assigning an electron- [CrL3] + + Fe2+ [CrL3] + + Fe + (81)... 

Redox potentials of chromium complexes Refractory metal electrodeposition Residual oxide in fluoride melts... 

In most natural waters at near neutral pH, Cr is the dominant form due to the very high redox potential for the couple Cr /Cr (Rai etal., 92>9). Chromium(III) forms strong complexes with hydroxides. Rai et al. (1987) report that the dominant hydroxo species are CrOH at pH values 4-6, Cr(OH)3 at pH values from 6 to 11.5, and Cr(OH)4 at pH values above 11.5. The OH ligand was the only signifrcant complexer of Cr in natural aqueous solutions that contain environmental concentrations of carbonate, sulfate, nitrate, and phosphate ions. The only oxidant in natural aquatic systems that has the potential to oxidize Cr ° to is manganese dioxide. This compound is common on Earth s surface and thus... 

The establishment of a solvent-independent reference electrode for the comparison of redox potentials in nonaqueous systems has a long story. It began with the concept of RblRb+ or Rb(Hg)IRb, followed by the proposal of using organometallic redox couples [2], In order to limit the number of redox systems used and then make the comparison easier, lUPAC recommended that the systems ferrocenelferrocenium, Fc° , and bis(biphenyl)chromium(0)lbis(biphenyl)chromium(l), BCi , need to be used as internal reference redox systems in nonaqueous media [2], These two complexes were selected arbitrarily from several published redox systems [2],... 

Table 6 Formal electrode potentials (V vs. SCE) for the redox processes exhibited by polypyridine complexes of chromium...

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

The chemistry of the transition metals including chromium(III) with these ligands has been the subject of a recent and extensive review,788 with references to the early literature. The close relationship between the catechol (180), semiquinone (181) and quinone (182) complexes may be appreciated by considering the redox equation below (equation 44). 789 The formal reduction potentials for the chromium(III) complexes (183-186 equation 45) are +0.03, -0.47 and -0.89 V (vs. SCE in acetonitrile) respectively. 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

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