Chromium redox chemistry

Masscheleyn, P. H., J. H. Pardue, R. D. DeLaune, and W. H. Patrick, Jr. 1992. Chromium redox chemistry in a lower Mississippi Valley bottomland hardwood wetland. Environ. Sci. Technol. 26 1217-1226. 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

This (reversible) reaction scheme allows much of the redox chemistry of chromium to be rationalized. 

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

Crumbie, R. L. Environmentally Responsible Redox Chemistry An Example of Convenient Oxidation Methodology without Chromium Waste, J. Chem. Educ. 2006, 83, 268-269. 

The Nozaki-Hiyama-Kishi (NHK) reaction involves the mild addition of chro-mium(II) organometallics to aldehydes to give homoallylic alcohols in a regio- and stereo-controlled fashion.111 A very significant achievement in the chromium organometallic chemistry was accomplished by Fiirstner who developed a catalytic version of the NHK reaction based on the coupled use of the redox Mn(0)/Cr(III) couple and trimethylsilyl chloride (TMSC1).[21 Moreover, the integration of the Fiirstner protocol with the addition of the Jacobsen s Salen /V,/V -bis(3,5-di-f-butylsalicylidene)-l,2-cyclohexanediamine] and triethylamine allowed Cozzi, Umani-Ronchi, et al. to develop a catalytic enantioselective route to homoallylic alcohols.[3]... 

For example, to replace chromium in corrosion protection, one must develop some new redox chemistry. 

Levina A, Mulyani I, Lay PA. 2007. Redox chemistry and biological activities of chromium(III) complexes. Jn Nutritional biochemistry of chromium(III). Ed JB Vincent. Amsterdam Elsevier Science. 

Several other useful reviews of reactions involving metal ions have also been published. Redox reactions of chromium(m)-amine species have been described and a survey has been made of the solution chemistry together with reaction paths involved in the redox reactions of various plutonium species. Oxidation reactions of thallium(m) have also been described. Developments in the redox chemistry of peroxides have been reviewed, the nature of the reactions which involve iron(iii) in various complexed forms providing a fascinating example of the manner in which geometry and co-ordination to the metal centre greatly affect the reactivity of the system. Redox properties of cobalt chelates, with delocalized... 

Chromium nitrosyl complexes, [Cr(S2CNR2)3(NO)], are somewhat less thermally stable than their molybdenum and tungsten counterparts heating [Cr(S2CNMc2)3(NO)] in toluene for 3 h leads to the formation of cis-[Cr(NO)2(S2CNMe2)2] and [Cr(S2CNMe2)3] (752). Somewhat contradictory reports siuround the electrochemical behavior of [Cr(S2CNR2)3(NO)]. Connelly and co-workers (752) report that they show no redox chemistry between 1.5 V,... 

Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

M. Rizzotto, A. Levina, M. Santoro, S. Garcia, M. Frascaroli, S. Signorella, L. F. Sala, and P. A. Lay, Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems, J. Chem. Soc. Dalton Trans. (2002) 3206-3213. 

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. 

A redox titration is based on an oxidation-reduction reaction between analyte and titrant. In addition to the many common analytes in chemistry, biology, and environmental and materials science thai can be measured by redox titrations, exotic oxidation states of elements in uncommon materials such as superconductors and laser materials are measured by redox titrations. For example, chromium added to laser crystals to increase their efficiency is found in the common oxidation states +3 and +6, and the unusual +4 state. A redox titration is a good way to unravel the nature of this complex mixture of chromium ions. 

The induction period can also be shortened or even eliminated by the addition of reducing agents either to the catalyst or to the reactor. Particularly effective are the alkyls or hydrides of aluminum, boron, zinc, lithium, magnesium, etc. When added in ppm quantities, they can eliminate the induction time of Cr(VI)/silica and also raise the steady-state polymerization rate. Some metal alkyls can remove poisons and redox byproducts. All metal alkyls no doubt help reduce the Cr(VI), perhaps to Cr(IV). And some may even help alkylate the chromium, similar to the chemistry of Ziegler catalysts. Figure 16 shows how triethylaluminum cocatalyst can be used to shorten the induction time [52],... 

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. 

---