System chromium

Na[AuClJ, per mole of silver haHde. Coordination compounds are used as emulsion stabilizers, developers, and are formed with the weU-known thiosulfate fixers. Silver haHde diffusion transfer processes and silver image stabilization also make use of coordination phenomena. A number of copper and chromium azo dyes have found use in diffusion transfer systems developed by Polaroid (see Color photography, instant). Coordination compounds are also important in a number of commercial photothermography and electrophotography (qv) appHcations as weU as in the classic iron cyano blueprint images, a number of chromium systems, etc (32). 

Plain carbon steels rust in wet environments and oxidise if heated in air. But if chromium is added to steel, a hard, compact film of CrjOj will form on the surface and this will help to protect the underlying metal. The minimum amount of chromium needed to protect steel is about 13%, but up to 26% may be needed if the environment is particularly hostile. The iron-chromium system is the basis for a wide range of stainless steels. 

Other more complex alloys based on the nickel-chromium system are the... 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

Since the theory of the mechanism of the microporous chromium system depends on the fact that the occlusions in the underlying nickel provide a... 

The ideal composition CrYI does not seem to be attained in all cases (see Table XII). Whether this is due to the existence of extended phases (e.g., CrSIi j) or represents an individual phase is not known. The compounds are described as black, and sensitive to air. Additional work should be done for the chromium system. 

In 1988, Trost and Tour published the cycloisomerization of an ene-allene using a nickel-chromium catalyst [134]. For example, 211 diastereoselectively led to 212 (Scheme 15.68). In the total synthesis of ( )-petiodial, this nickel-chromium system failed, but a palladium catalyst was successful [135]. 

The general approach is illustrated in detail for the case of aqueous ferrous and ferric ions, and the calculated rate constant and activation parameters are found to be in good agreement with the available experimental data. The formalisms we have employed in studying such complicated condensed phase processes necessarily rely on numerous approximations. Furthermore, some empirical data have been used in characterizing the solvated ions. We emphasize, nevertheless, that (1) none of the parameters were obtained from kinetic data, and (2) this is, as far as we are aware, the first such theoretical determination to be based on fully Ab initio electronic matrix elements, obtained from large scale molecular orbital (MO) calculations. A molecular orbital study of the analogous hexaaquo chromium system has been carried out by Hush, but the calculations were of an approximate, semi-empirical nature, based in part on experi-... 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

An interesting chromium system example is represented by complex 145. Addition of cyano-Gilman cuprates occurred with complete diastereoselectivity to give conjugate adducts 146 (Scheme 6.28). Interestingly, the opposite diastereomer was accessible by treatment of enone 145 with a titanium tetrachloride/Grignard reagent combination [71c]. 

John P. Hunt So far we have not seen any organic free radicals in these experiments. We have looked mainly at the maleatopentammine Co(III) system s reacting with chromium(II) and vandium(II). In the chromium system this problem is complicated by a large chromium(III) ESR signal. In the case of vanadium, we sometimes see small traces of vandium(IV) form which we don t completely understand. 

I have one final comment on the chromium system. There is another doublet state, a doublet that is supposed to be present and usually hidden by the quartet-quartet transitions. 

Figure 1.1.8 illustrates the aluminum (39) and chromium (hydrous) oxide (55) particles prepared under conditions given in the legend. Some details of the mechanisms of formation for the chromium system are given in Section 1.1.3.2. 

Structural data confirm that the Daniphos ligands are readily adaptable to most ligand environments. A major advantage of the chromium system is the commercial availability of phenylethylamine and its derivatives in both enantiomeric forms. In contrast to the cyclopentadienyl ring, arenes have an almost unlimited potential for controlled substitutional variation, accessible through standard reaction protocols. 

M(IV)-peptide radicals, and decay to final products have been determined (96). Interestingly, the rate constants for both the formation and decay of the high-valent intermediate are much larger than the corresponding values in the chromium system in Scheme 1. Thus the lifetimes of both the hydroperoxo species and M(IV)-peptide radicals are much shorter in the enzymatic system than for the chromium macrocycles. 

Replacement of hexavalent chromium with trivalent chromium offers important environmental advantages. Trivalent chromium is considerably less toxic than hexavalent. Trivalent systems use chromium concentrations that are typically two orders of magnitude less than in hexavalent systems. Thus, far less chromium enters the waste stream. Trivalent systems also generate few toxic air emissions, while hexavalent systems involve a reaction that produces hydrogen bubbles which entrain chromium compounds and carry them out of the baths. Trivalent chromium is readily precipitated from wastewater, while hexavalent chromium solutions must go through an additional step in a treatment system in which the chromium is reduced to its trivalent form before precipitation. It has been shown that trivalent chromium systems can successfully replace hexavalent ones for decorative chrome applications. Trivalent chromium systems are not suitable for hard chrome applications. More information regarding trivalent chromium plating can be obtained from Roy (1984), Robison (1978), Chementator (1982), and Smart (1983). 

In this sector of the market nickel-chromium systems are used most widely but other finishes have become fashionable, including gold for luxury bathrooms. Generally in this instance a thin coating of gold is provided over thicker layers of copper and nickel (similar to those used beneath chromium). An important feature recently was the introduction of the special filled poly(bu-tylene terephthalate) mentioned in an earlier section, which conveys a sense of weight and quality well-suited to products with more costly finish. 

Another important application is the decoration of buttons the annual production in Britain of plated buttons is estimated at between 100 and 150 millions. Usually the processes employ barrels rather than mounting the buttons on jigs, with application of nickel-chromium systems however, finishes such as brass, bronze, and gold also are popular. 

Complex 2 is a potent cluster precursor. The isolobal relationship (194) between diarylalkynes and the alkylidynes CpW(CO)2(=CR) is now quite well known (195). Since 2 and 3 readily add alkynes, one might reason that alkylidynes CpW(CO)2(=CR) will also add across the M=M bonds of 2 or 3. In fact, this happens, and in quantitative yield for 2, when the product is 73. The yield of 74 in the tungsten reaction is less impressive, but it is obtained in higher yield in the reaction of Cp2W2(CO)4-(fi-rf-RCCR) with CpW(CO)2(=CR). The behavior of the chromium system is quite different, the product being the /x-alkyne species Cp2-W2(CO)4(jU-i72-RCCR). Remarkably, 1 is catalytic for this dimerization (196). 

Iron-chromium alloys, free from carbon, may be prepared from chromite by the alumino-thermic method. From a study of the cooling-and freezing-point curves it has been suggested that a compound, Cr Fe, exists, but this is questioned by Janecke, who studied the iron-chromium system by means of fusion curves and by the microscopic study of polished sections of various alloys between the limits 10 Fe 90 Cr and 90 Fe 10 Cr, and came to the conclusion that the system consists of a single eutectic which can form mixed crystals with either component. The eutectic contains 75 per cent, of chromium and melts at 1320° C. The addition of chromium to iron increases the readiness of attack by hydrochloric and sulphuric acids, but towards concentrated nitric acid the alloys are rendered passive. They remain bright in air and in water. The presence of carbon increases the resistance to acids and renders them very hard if carbon-free, they are softer than cast iron. All the alloys up to 80 per cent, chromium are magnetic. Molybdenum, titanium, vanadium, and tungsten improve the mechanical properties and increase the resistance to acids. 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

As an example, the experimental results for the anodic dissolution of alloys of the nickel-chromium system are presented in Figs 5 and 6. This composition is the basis of several superalloys, which are machined using the ECM. Alloy components - chromium and nickel - exhibit different tendencies to passivate. Nickel is weakly passivated in the NaCl solution its tendency to passivate is much stronger... 

The procedure is discussed in detail by Delmon et al.(82-84). The crucial step appears to be the rapid dehydration of the starting solution before any of the components can crystallize out of solution separately. Delmon(85) suggests that a rotary vacuum evaporation would be an effective method of drying the precursor. The actual structure of the precursor is not well defined, but appears to require at least one equivalent of citrate ion per mol of metal ion(83), as presumably the citrate complexes all the metal species in so Iution. The resulting powder patterns, after annealing, indicated no contamination. Delmon(J3) suggests that any multifunctional acid containing at least one carboxyl and one hydroxyl function may be effective. Experiments with tartaric acid on the iron/chromium system produced results similar to citric acid a calcination temperature of 500°C was necessary before crystallization occurred. 

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