Halide complexes chromium

Dehalogenation. Barton et at. (1, 148) effected dehalogenation of steroidal /i-hydroxy halides with chromium(II) acetate and butancthiol as the proton donor in DMSO. The method is only useful with tertiary halides. A recent improvement that permits reduction of halides of all types uses the ethylenediamine complex of CrtCIOzh and the tetrahydropyranyl ethers of the /J-hydroxy halide. Catalytic amounts of the reducing agent can be used in "indirect electrolysis." The reaction is convenient for preparation of deoxynucleosides.1... 

The general formula [RCr(OH2)s] + describes this group of complexes. Reaction of an alkyl halide with chromium(II) perchlorate in perchloric acid yields these... 

Studer P (1975) Optical studies on chromium (IV) halide complexes, Ph.D. Thesis, University of Fribourg, Switzerland... 

The tendency of halides to form metal halide complex is very important in understanding the stabilization of corrosion pit by prevention of the repassivation of a defect site within the passive layer. Among the halides, fluoride forms strong complexes with metals. The resistance of chromium to localized corrosion is because of slow dissolution kinetics of Cr(III) salts. Higher-valence oxides are the best passivators (films) because of their slow rates of dissolution. 

Halides. Kinetics and mechanisms of aquation of chromium(in)-amine- or -ammine-halide complexes are discussed in an extensive review of the... 

Acyclic a,a-disubstituted tin enolates 6 inevitably form as cis/trans-mixtures. Nevertheless, application of the chromium alkylation protocol with the modified salen complex 7 provides fair enantioselectivity with various alkylating agents R CH2X hke allyl bromide, benzyl bromide, allyl iodide, and ethyl iodoacetate, as outlined in Scheme 5.5. A plausible explanation is based on the assumption of a rapid cis/trans-isomerization of the tin enolates 6 through the C-bound tautomer and the postulate that one of the enolate diastereomers reacts distinctly faster than the other. The role of the additive BugSnOMe, which has a beneficial effect on the enantioselectivity, might be to catalyze the cis/trans-isomerization of the enolate. Several models have been proposed for the mechanisms of the enantioselective enolate alkylation like transmetallation of tin into a chromium enolate, formation of a stannate by iodine transfer from chromium to tin, as well as activation of the alkyl halide by chromium [5]. 

Due to the stability of intermediate complexes between the metal substrate and the aggressive anions, pitting corrosion does not occur for chromium metal. Stability constants of CrX complexes are smaller than 1, for instance it is 1 when X is C1 and lO" when it is 1. In addition, exchange of CT and HjO ligands between the inner and outer sphere of chromium halide complexes is extremely slow. Together these factors causes insolubility of CrClj in cold water due to very low dissolution rate of Cr. Therefore the presence of a Cr-Cl complex at the surface will not increase the dissolution rate because it will dissolve very slowly by itself. In the case of this exchange is very rapid. Similarly Fe-Cr alloys are more resistant to pitting in Cl" solution than is pure Fe. 

Whereas the assignment of mechanism to spontaneous thermal aquations may at times be uncertain, the mechanism of metal-ion-catalysed aquation of halide complexes of cobalt(iii), chromium(ni), and similar cations is unlikely to be other than dissociative as far as the metal(m) centre is concerned. In Volume 2 of this Report it was mentioned that the catalytic effect of metal ions on solvolysis rates of t-butyl halides could be correlated with the stability constants of the respective metal-halide complex formed. Such a correlation is now reported for metal-ion catalysis of aquation of halide complexes of cobalt(m), chromium(m), and rhodium(m). Indeed this correlation is sufficiently general as to embrace such catalysts as H+ and HgCl+ as well as metal ions such as Hg + and A linear free-energy (AG vs. AG°) correlation... 

Kinetic studies of the loss of ammonia or of chelating amines from cobalt(m) complexes by aquation are rare. Parallel loss of ammonia and halide from chromium(m)-ammine-halide complexes is not uncommon, but ammonia is very much more reluctant than halides to depart from cobalt(ra). In fact the potential field of kinetic studies of the loss of ammonia from cobalt(m) is small, and further restricted by the propensity of cobalt(m) to oxidize co-ordinated water. The [Co(NH3)(OH2)5] + cation, on warming in IM-HCIO4 for twelve hours, gives [Co(NHa)2(OH2)4] + and cobalt(ii). The... 

The rate laws for aquation of the complexes [Cr(OH2)6X] +, where X = Cl, Br, I, NCS, or NOg, include acid-dependent and acid-independent terms. Transition enthalpies (Affx, the enthalpy difference between transition state and products) have been determined for the acid-independent path of these aquations. The Ht values cover a range of 6.4 kcal mol. The size of this range and the order of values within it both suggest that the leaving group is less solvated and less dissociated from the chromium than in aquations of cobalt(m)-ammine-halide complexes. ... 

Halide and oxyhalide complexes of elements of the titanium, vanadium and chromium sub-groups. 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

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