Other Chromium Oxidation States

The kinetics of the Zn -induced dissociations of [CrL3] (L 4,4 -diMe-bipy and 5,5 -di-Me-bipy) have been studied.  

Electron spin resonance evidence for the formation of Cr(V)-nitrides was described previously.  

Substitution Reactions of Inert Metal Complexes— Coordination Numbers 6 and Above Cobalt 

The level of activity in this area remains high, with a considerable volume of interesting work appearing, particularly in the area of the reactions of coordinated ligands. Review articles of interest include a general discussion of the chemistry of cobalt(III) complexes and a review dealing with activation volumes and their application to the determination of reaction mechanisms.  

Lawrence has recently reported activation volumes for the aquation of alcoholpentaamminecobalt(III) complexes (see Table 7.1). Partial molar 

The kinetics of the dissociation of [Cr(LL)3] (LL = bipy or phen) were considered previously.  

ESR and visible spectroscopic evidence has been found for chromium(V) intermediates during the oxidation of [Cr(L)(OH] (L = hydroxyethylenediaminetriacetate or edta) and [Cr(LL)2(OH)2] (LL = ox, mal) with H202. Oxidation of [Cr(Hedta)(OH2)] with IO4 to Cr04 has also been studied/ The photochemical or thermal formation of chromium(V) complexes with crown ethers is reported from reactions of Cr207 in nonaqueous solvents in the presence of crowns.  

Studies of oxygen exchange between [Cr(NH3)50Cr03] and H2O at pH 6.7 shows that Cr-0 bond fission is involved in the loss of the chromate ion from the inner sphere of the cobalt(III).  

The formation of the chromium(VI) violet diperoxo complex, [H0Cr(0)(02)2] parallels that found for the blue [H20Cr(0)(02)2] in being a third-order process. Activation parameters for the rate-determining step are AH = 6.6 k cal mol and AS = -14 cal K mol .  

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Figure 9-19. In the case of chromium(vi) and other high oxidation state alkoxy complexes, the C-H bond breaking may occur in an intramolecular two-electron process.

Carbonyl and other Low Oxidation State Compounds.—Tris-(2,2 -bipyridyl)vana-dium(O) solutions in DMF exhibit three one-electron reduction waves corresponding to VL3 - VL3 (L = 2,2 -bipyridyI) VLJ -> VL " and VLf -VL, and two oxidation waves corresponding to VL3-> VL3 and VLJ ->VL3+ whose half-wave potentials differ by only 0.10 V.342 The difference for the isoelectronic chromium complexes is 0.49 V. An explanation has been offered for the small difference between the vanadium potentials in terms ol the possible diamagnetism of the d4VL3 species arising from a substantial splitting of the l2g orbitals. 

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Chromium Exposure Levels and U.S. Government Regulations. The level of exposure to chromium compounds for employees in industry and for the general population via waste disposal and industrial emissions is the subject of much regulation, research, and controversy. Some U.S. Government regulations, such as the Comprehensive Environmental Response, Compensation, and LiabiUty Act (CERCLA), also known as the Superfund Act, make no distinction as to the oxidation state of chromium (144). However, there is valence distinction in other regulations. 

Other ions which are reduced in the reductor to a definite lower oxidation state are those of titanium to Ti3+, chromium to Cr2+, molybdenum to Mo3+, niobium to Nb3+, and vanadium to V2 +. Uranium is reduced to a mixture of U3 + and U4+, but by bubbling a stream of air through the solution in the filter flask for a few minutes, the dirty dark-green colour changes to the bright apple-green colour characteristic of pure uranium(I V) salts. Tungsten is reduced, but not to any definite lower oxidation state. 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

Hg " complexes are common, but complexes of Na, Mg", or Al are rare. Chromium complexes are also common, but in such complexes the chromium is in a low or zero oxidation state (which softens it) or attached to other soft ligands. In another application, we may look at this reaction ... 

Cr3+ and Cr6+ are the most stable oxidation states of chromium, but with the only difference that while +3 oxidation state is cationic where as the +6 oxdation state is oxoanionic. However, the other oxidation states of +1, +2, +4 and +5 are also known for chromium, especially in aqueous solution at different pH. Inter-conversion of these oxidation states too is very frequent. With this view, an attempt is made here to examine the effect of ultrasound on the inter-convertibility of chromium among various oxidation states in aqueous solutions. The details of this study is reported in the literature [36]. 

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

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