Chromate tris

Chromate, tetrakis(dioxygen)-stereochemistry, 94 Chromate, tricyanodiperoxy-structure, 78 Chromate, tris(oxalato)-racemization solid state, 466 strychnine salt racemization, 466 Chromatography optical resolution, 26 Chrome azurol S metallochromic indicator, 556 Chromium... 

Hydrated forms of the hydroxide ion have been much less well characterized though the monohydrate [H302] has been discovered in the mixed salt Na2[NEt3Me][Cr PhC(S)=N-(0) 3]. NaH302.18H20 which formed when [NEt3Me]I was added to a solution of tris(thiobenzohydroximato)chromate(III) in aqueous NaOH. ° The compound tended to lose water at room temperature but an X-ray study identified the centro-symmetric [HO-H-OH] anion shown in Fig. 14.15. The central O-H-O bond is very short indeed (229 pm) and is... 

The CIELAB system (1976) strictly standardizes the light source and the observer. CIE recommends three standard sources, A is an incandescent lamp, and B and C are lamps provided with different two-cell Davis-Gibson liquid hlters that simulate noon daylight and average daylight, respectively. Since the main object of the system is to obtain colorimetric results for normal tri-chromats (people with normal color vision), the standard observer must represent the human population with normal... 

Chromium hexavalent, the more toxic than the tri-valent form, comes in surface waters from anneries, chemical, energetic and ceramic industries. The. stable chromate form predominates in alkali, whereas dichromate in acidic waste waters, mostly in deficiency of reductives.6... 

Photolysis of triarylsilyl chromates in the presence of PBN gives triarylsilyl adducts by way of what was considered to be the first example of the transformation of Cr(VI) into Cr(V) (Rehorek et al., 1978). Photolysis of a tris(oxalato)iron(III) complex with PBN gives species considered to be the adducts of (C02)2 and C02 (Rehorek et al., 1977). 

Chromium-mediated oxidation of benzylic methylene groups to the corresponding oxo derivatives has also been reported [4], Tri-n-butylstannyl chromate appears to be the best co-oxidant and a trace of 4-toluenesulphonic acid also aids the oxidation. 

A review by Galli et al. describes several buffer-absorbing chromophores as co-ions. These include phthalate, PDG (2,6-pyridinedicarboxylic acid), PMA (1,2,4,5-benzenetetra-carboxylic acid or pyromellitic acid), TMA (trimellitic acid), MES, 2,4-dihydrobenzoic acid with s-aminocaproic acid, p-hydroxybenzoate, p-anisate, 3,5-dinitrobenzoic acid, salicylic acid with TRIS, benzoic acid with tris (hydroxymethyl)aminomethane (TRIS), and many others. On the other hand, some inorganic chromophores such as chromate (Figure 9) or molybdate may be added to a buffer. A BGE-containing chromate should have a pH above 8, because it precipitates below this value. The advantage of a TRIS buffer or buffers at around pH 6 is that carbonate will not interfere with the separation because it is not soluble in TRIS or at lower pHs. 

None of these things happened when we tried this reaction. What did happen —and it took some puzzling to figure it out—was that it reacted with a nitrate ion as the solvent to give two chromate ions plus nitryl ion, and the nitryl ion made nitryl bromide which decomposes to bromine and N02. Incidentally, we found that... 

CrFeN2P4C8lH , Chromate(2-), nona-carbony liron-bis[ p-ni trido-bis( tri-phenylphosphorus(l +)], 26 339... 

When sodium chloride is fused with anhydrous chromium(II) chloride the product is NaCrCI5 regardless of the proportions of chlorides used.222 Other cations give tetra- or tri-chloro-chromates(II) as in Sections 35.3.7.3.ii and 35.3.7.3.vi above. The solvate [pyH]3[CrBr5]-2MeC02H has been isolated from the metal acetate and pyridine in a mixture of acetyl bromide and acetic acid, and there is considerable splitting of the spin-allowed d-d band in the reflectance spectrum of this complex,255 but no detailed investigations of pentahalochromates have been reported. 

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

Oxalate complexes of chromium(III) were first characterized at the turn of the century by Rosenheim and Cohen.877 The most extensively studied are the tris species and the cis- and frarw-bisoxalates (205-207) these formulations were first suggested by Werner.878 All may be made by the reduction of chromate with oxalate. Reliable preparations have been reported for tris by Kauffman and Faoro879 and for cis- and rrans-diaquabisoxalatochromateflll) by Bailar.880... 

Table 81 The Electronic Spectrum of Potassium Tris(oxalato)chromate(III) (after ref. 889)...

Malonic acid CH2(C02H)2 (H2mal) (209) has a coordination chemistry with chrommm(III) closely resembling that of oxalate. Malonic acid is a slightly weaker acid than oxalic acid and slightly more labile complexes are formed. The tris complex is the most extensively studied, prepared by the reduction of chromate solutions or the reaction of chromium(III) hydroxide with malonate.917,918 919 The cis and trans diaqua complexes may be prepared by the reduction of chromate with malonate the isomers are separated by fractional crystallization. The electronic spectrum of the tris complex is similar to that of the tris oxalate and a detailed analysis of these spectra has appeared.889... 

The structures of the tri- and tetra-chromates, e.g. Rb2Cr3Oi0 (294) and Rb2Cr4Oi3 (295), and of polymeric Cr03 (296) follow the same general pattern, although the detailed structure can vary with the cation. 

Double sulphate precipitation is one of the most common methods used in industry for the separation of cerium group from yttrium group rare earths. Various other precipitants such as chromates, double chromates, ferrocyanides, phosphates etc. have been tried. 

It is important to note that the relative velocity of an uneventful oxidation of an alcohol with PCC versus a carbon-carbon bond breakage from a chromate ester, driven by the generation of a stable carbocation, is substantially substrate-dependent, and may change according to stereoelec-tronic factors, which may be difficult to predict. Thus, many alcohols are successfully oxidized to aldehydes and ketones, regardless of an apparently potential carbon-carbon bond breakage leading to stabilized carboca-tions.315 Consequently, failure to try an alcohol oxidation with PCC, because of fear of this side reaction is not recommended. 

Aside from these three classes (species with unfilled inner subshells, with unpaired electrons, or with two different oxidation states of the same element), there are a number of colored inorganic substances about which generalizations may be set up only with difficulty. Among these are many of the elementary nonmetals, a large number of covalent salts (such as mercuric iodide, cadmium sulfide, silver phosphate and lithium nitride), a number of nonmetal halides (iodine monochloride, selenium tetrachloride, antimony tri-iodide, etc.), and the colored ions, chromate, permanganate, and Ce(H20) v, whose central atoms presumably have rare-gas structures. 

Diaquabis(oxalato)chromates(III) may also be prepared by the reduction of chromium(VI) oxide by oxalic acid,7 aquation of the tris(oxalato)-chromate(III) ion,8,9 and cleavage of the [(C204)2Cr(0H)2Cr(C204)2]4-ion.10 The mechanism of their formation from hexaaquachromium(III) and oxalate ions11 has been established by polarographic,1213 conductometric,12 and spectrophotometric1314 measurements. 

---