Chromium anion

In molecular orbital terms, the donation can be viewed as a HOMO-LUMO interaction (Chapter 9). Double hydride bridges, as found in B,H, also are exhibited by bimetallic species such as the chromium anion formed in Eq. 15.18. The sirmlarnv between these two dibridged species is underscored by the fact that their Lewis acid and Lewis base fragments can be interchanged [HCr(CO)sJ reacts with BH, to form 16... 

This variance in intimate mechanisms is likely seen in the reactions of HCr(CO)5 and HW(CO)j with C02 to provide HC02M(C0)j derivatives (45). For example, carbon monoxide dissociation in the chromium anion, as evinced by l3CO exchange studies, occurs at a rate indistinguishable by conventional techniques from that of C02 insertion. Consistent with this observation, the rate of decarboxylation of HC02Cr(CO)jis retarded in an atmosphere of carbon monoxide. Similar behavior was noted in decarboxy-... 

The presence of 1 equivalent of TPP1 OTf- with the chromium anion TpCr(CO)3- as the tetrabutylammonium salt in dichloromethane results in the loss of the carbonyl bands of the anion at 1890 and 1740 cm-1. Their complete replacement by the sharps, band at 2018 cm-1 and the broad E band (1898 and 1838 cm-1) of the 17-electron radical TpCr(CO)3- indicates that the ion-pair annihilation proceeds to completion. Variation of the pyrylium cation, by the replacement of TPP+ with a weaker acceptor such as tri-p-anisylpyrylium triflate (TAP+ OTf-), consistently results in lower conversions of the carbonylmetal anions. For example, the treatment of TpMo(CO)3 with the TAP+ salt leads to a light red solution of TAP (Am 560 nm) (92) and a greatly diminished concentration of TpMo(CO)3- as judged by the reduced carbonyl absorbances in comparison with that obtained from TPP+ at the same concentration. Even with this weaker acceptor cation, however, the strong chromium anionic donor TpCr(CO)3- is completely oxidized by 1 equivalent of TAP+ to form TpCr(CO)3- in essentially quantitative yields. 

Zouboulis, A.L, Kydros, K.A., and Matis, K.A., Removal of hexavalent chromium anions from solutions by pyrite fines. Water Res., 29, 1755, 1995. 

Attempts to isolate GTF from brewer s yeast have resulted in production of very active concentrates, but the substance is too labile to be obtained in the soHd state (136). However, it has been shown that GTF is a Cr(III) complex containing two coordinated nicotinate radicals and other amino acid anions (146). Active preparations containing similar complexes have been synthesi2ed (147). Chromium deficiency may also lead to atherosclerosis and peripheral neuropathy. 

The hydrolysis equilibria for H2Cr04 given in Table 3 are only valid in HNO or HCIO solutions. Other acids yield complexes such as those shown for chloride and bisulfate ions. The exact composition of chromate(VI) anion(s) present in aqueous solution is a function of both pH and hexavalent chromium concentration (68). However, at pH values above 8, virtually all the Cr(VI) is present as the CrO anion. When the pH is between 2 and 6, an equilibrium mixture of HCrO and Ci2 is present when the pH is below 1, the principal species is H2Cr04 (68,69). At very high Cr(VI) concentrations... 

Although equation 9 is written as a total oxidation of sugar, this outcome is never realized. There are many iatermediate oxidation products possible. Also, the actual form of chromium produced is not as simple as that shown because of hydrolysis, polymerization, and anion penetration. Other reduciag agents are chosen to enhance the performance of the product. 

The compositions of the conversion baths are proprietary and vary greatly. They may contain either hexavalent or trivalent chromium (179,180), but baths containing both Cr(III) and Cr(VI) are rare. The mechanism of film formation for hexavalent baths has been studied (181,182), and it appears that the strength of the acid and its identity, as well as time and temperature, influences the film s thickness and its final properties, eg, color. The newly prepared film is a very soft, easily damaged gel, but when allowed to age, the film slowly hardens, assumes a hydrophobic character and becomes resistant to abrasion. The film s stmcture can be described as a cross-linked Cr(III) polymer, that uses anion species to link chromium centers. These anions may be hydroxide, chromate, fluoride, and/or others, depending on the composition of the bath (183). 

Acid—mordant dyes have characteristics similar to those of acid dyes which have a relatively low molecular weight, anionic substituents, and an affinity to polyamide fibers and mordant dyes. In general, brilliant shades caimot be obtained by acid—mordant dyes because they are used as their chromium mordant by treatment with dichromate in the course of the dyeing procedure. However, because of their excellent fastness for light and wet treatment, they are predominandy used to dye wool in heavy shades (navy blue, brown, and black). In terms of chemical constitution, most of the acid—mordant dyes are azo dyes some are triphenyhnethane dyes and very few anthraquinone dyes are used in this area. Cl Mordant Black 13 [1324-21 -6] (183) (Cl 63615) is one of the few examples of currentiy produced anthraquinone acid—mordant dyes. It is prepared by condensation of purpurin with aniline in the presence of boric acid, followed by sulfonation and finally by conversion to the sodium salt (146,147). 

Chromium is conventionally deposited from chromic acid solutions containing at least one anionic catalyst, which is usually the sulfate ion. The weight ratio of chromic acid to catalyst is important and, for sulfate-cataly2ed solutions, is maintained about 100 1. Formulations and conditions for operating hard chromium plating solutions are shown in Table 5. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

Chromium(III) forms stable salts with all the common anions and it complexes with virtually any species capable of donating an electron-pair. These complexes may be anionic, cationic, or neutral and, with hardly any exceptions, are hexacoordinate and octahedral, e.g. ... 

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