Chromium complexes solution studies

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

The existence of such complexes was first postulated by Bjerrum in 1908.705 There have been a number of studies in recent years. Several early members of the series of hydrolytic polymers formed on the addition of base to aqueous chromium(III) solutions have been isolated 706 purification was achieved by ion-exchange chromatography on Sephadex-SP C25 resin. The structures suggested for these complexes are illustrated (156-162). The structural unit (162) was held to be singularly important, both as a constituent of higher polymers and in the mechanism of dimerization. Equilibrium data for these complexes are summarized in Table 78. 

Exposure to chromium(VI) can result in DNA-protein complexes, the identification of which may be useful as biomarkers of exposure to chromates (Costa 1991). Gel electrophoresis and immunochemical techniques were used to identify actin as the protein in a DNA-protein complex induced by potassium chromate in cultured Chinese hamster ovary cells. While the DNA-protein complexes induced by formaldehyde and ultraviolet light were different from those induced by chromate, actin was also identified as the protein in the complex induced by cis-platinum, indicating that the DNA-actin complex is not specific for chromium. However, an experiment in a group of four volunteers did not demonstrate an increase in DNA-protein crosslinks in leukocytes over a 240 minute period following the ingestion of 5 mg chromium(VI) as potassium dichromate in a 10 mg chromium/L solution or the same amount added to 300 mL of orange juice (presumably reducing chromium(VI) to chromium(III)) and diluted to 500 mL with deionized water (Kuykendall et al. 1996). Chromium levels in red cells, plasma and urine were increased. In a separate experiment in this study, a threshold dose of 52 pg chromium(VI)/L was determined for crosslink formation in cultured lymphoma cells. 

Numerous salts of [Cr(bpy)3]2+ have been prepared by reaction of chromium(II) solutions with excess bpy in inert conditions the complexes are paramagnetic and readily oxidized to [Crfbpyls] (33, 567, 621, 905, 909, 918). Studies of the NMR spectra of [Cr(bpy)3] + salts have given some information on their electronic structure (527, 528). 

Thermal decomposition of hydrated (Li and Na) and anhydrous (K and Pb) alkali ferricyanides has been studied up to 650 °C. The presence of water lowers the decomposition temperature and may also cause formation of FejOs at higher temperatures. In the case of the anhydrous complexes, the end product is iron, whereas with the hydrated species hydrated alkali ferrites are formed. ° [(NQgFe—CN—Cr(NH3)4L] (L = HjO or NH3) have been prepared from the appropriate chromium complex and Fe(CN)e Exposure of methanolic solutions of [(NQ5FeN(Ph)0] - to °Co y-rays at 77 K yields [(NC)5FeN(Ph)OH]3-.2 52... 

Reductions of a series of cobalt(iii) complexes, [CoX(NH3)5], with chromium(II) solutions were studied. Transfer of the X group to chromium occurs when X is NCS, N3 , P04, C2H3O2, Cl, Br , and 864 (46). This suggests that all of these reactions are of the inner-sphere type. 

The decomposition of cyclohexylhydroperoxide was also studied in the presence of molybdenum and chromium complexes [356]. The decomposition of cyclohexylhydroperoxide in benzene catalyzed by [Mo02(acac)2], has many characteristics of the [VO(acac)2]-catalyzed reaction [355]. The ketone/alcohol ratio in the product was 1 and the kinetic pattern of reaction is similar. When chromium(III) acetylacetonate is used, however, there is a substantial difference. The chromium complex selectively converts cyclohexyl hydroperoxide to cyclohexanone. It is suggested that in this case the extent of release of free radicals to the solution is small [356]. The ketone/alcohol ratio in this case is " 13.7. The predominant formation of cyclohexanone on decomposition of cyclohexyl hydroperoxide in the presence of [Cr(acac)3] is no doubt related to the much higher yield of ketone obtained in cyclohexane oxidation in the presence of chromium complexes than observed when Mo or V compounds are used as catalysts [356]. 

Solutions of the complexes studied were prepared in reagent grade benzene. The solvent was deaerated prior to solution preparation to remove all traces of dissolved oxygen and thus minimize oxidation of the chromium complexes which occurs much more readily in solution than in the solid state. The greater solubility of the complexes in benzene as compared to saturated hydrocarbons, such as hexane, made it a more useful solvent. 

Johnson et al. studied analogs of CP in which the cobalt atom was replaced by chromium, iridium, rhodium, and ruthenium. The rhodium and chromium complexes can be prepared by reaction of the APCP analog [Me(NH3)5(H20)] ( 104)3 with 5-cyanotetrazole in an aqueous solution, whereas the others have been shown to consist of product mixtures. The rhodium complex is the most thermally stable (comparable to CP) while the iridium, chromium, and ruthenium complexes are significantly less stable (determined by DSC) [4, 25]. 

The nitrogen complex had already been synthesized in a solid matrix, but its decomposition kinetics and its further photolysis could be studied only in solution. The liquid noble gas technique is superior to the solid matrix technique, especially for the synthesis of multiple substituted chromium carbonyl nitrogen complexes. Their IR spectra were extremely complex in matrices, due to "site splittings" which arise when different molecules are trapped in different matrix environments /18/. 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

HCrO ] has been shown to be the photo-active species in the photochemical reaction of dilute solutions of chromium(vi) oxyacids with alcoholic reducing agents. The association of chromate with Np , Th, and Fe" in perchlorate solution has been studied spectrophotometrically and the respective formation constants, 63.6,4.70, and 1.93, have been determined. The greater stability of the Np complex has been interpreted in terms of the limited donation of 5/-electron density from Np to the d-orbitals of chromium in the chromate ion. ... 

R-dtp complexes in solution 1 1.397-399) have been described but the complexes have not been carefully characterized. Comparable studies have also been reported ° ) for chromium(V) and molybdenum(V) complexes of dithizon. 

In summary, spectroscopic studies show that at low temperatures higher excited states of chromium(III) complexes undergo rapid internal conversion to the metastable T2g or 2Eg levels. Intersystem crossing from the 4T2g to the aEg level occurs with near unit efficiency in many chromium(III) complexes. Phosphorescence competes, usually unfavorably, with radiationless decay from the 2Eg state. Studies of the excited states of Mo(III), (4d)3, based on absorption spectra of its complexes in solution, have recently been reported.134-137... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

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