Chromium divalent

Group 6 (VIB) Perchlorates. Both divalent and trivalent chromium perchlorate compounds [13931 -95-8 13527-21 -9] have been reported. Anhydrous chromyl perchlorate [60499-74-3] has been prepared ia the cold ... 

Our attempts to prepare chromium hydrides and to evaluate their role in polymerization catalysis eventually led to the isolation of a series of alkyls and hydrides lacking any ancillary ligands besides the cyclopentadienyl moiety (see below).[6] Reduced to the essence of alkyls, these complexes provided another piece of evidence in the growing case against polymerization activity of divalent chromium none of the alkyls even reacted with ethylene. The hydride underwent one insertion and stopped at the stage of an ethyl group. 

The alkyls Tp Cr-R are the best test case yet of the catalytic activity of CrU alkyls (see Section 1). However, they did not react with ethylene, even at elevated temperature. On the contrary, Tp - Cr-Et eventually decomposed by an apparent P-hydrogen elimination yielding Tp - Cr-H and ethylene. Thus our notion that divalent chromium alkyls are not the chain propagating species in polymerization catalysis receives further support... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

As a practical example, following the reduction of hexavalent chromium, sodium hydroxide, lime, or sodium hydroxide can be added to the wastewater to neutralize the pH and precipitate the trivalent chromium, nickel, iron, divalent, and other heavy metals. If lime is used, lime will react with heavy metals and with any residual sodium sulfate, sulfurous acid, or sodium bisulfite. The following reactions apply ... 

As stated in the introduction, hexamethyldisilazyl can be used to coordinate electropositive elements in a very efficient way. Nevertheless, with divalent metallic elements it is often found that further organic bases are coordinated to the metal thus, in [(Me3Si)2N]2Cr(THF)2 the chromium atom is linked to two nitrogen and two oxygen atoms [9]. Such compounds tend to dissociate in the gas phase which makes them less appropriate for reactions controlled by pressure and temperature, as in MO-CVD processes. 

The chemistry of aluminium combines features in common with two other groups of elements, namely (i) divalent magnesium and calcium, and (ii) trivalent chromium and iron (Williams, 1999). It is likely that the toxic effects of aluminium are related to its interference with calcium directed processes, whereas its access to tissues is probably a function of its similarity to ferric iron (Ward and Crichton, 2001). The effective ionic radius of Al3+ in sixfold coordination (54 pm) is most like that of Fe3+ (65 pm), as is its hydrolysis behaviour in aqueous solution ... 

Acute toxicides of hexavalent and divalent chromium to aquatic life... 

Maximum acceptable toxicant concendation (MATC) values for hexavalent and divalent chromium to aquatic life based on life cycle or partial life cycle exposures... 

Water-soluble divalent and trivalent chromium compounds <0.5 7... 

With the divalent chromium(II) ion there are two distinct possibilities, shown in lines (c) and (d). With such an isolated ion having four electrons in the d shell the fourth electron may occupy either an eg level (c) or a t2g level (d), depending on whether the energy required to cause spin pairing is less or greater than the difference between these energy... 

Burns R. G. (1975). On the occurrence and stability of divalent chromium in olivines included in diamonds. Contrih. Mineral. Petrol, 51 213-221. 

The spectrum of the organometallic compound dimesitylene chromium, [C Hj(CHs)3]2Cr, has relatively high amplitude. The location of the principal peak, 18 ev., corresponds to that of divalent chromium. 

The other end of the series, K3[Cr(NCS)e], has a spectrum with a pronounced preliminary peak at 16 ev., which peak increases in magnitude throughout the series in Fig. 18. The location of the peak might imply divalent chromium. However, from diffraction type arguments and the interpretation of the K3Co(CN)e spectrum of Fig. 13, this new peak introduced by the NCS group may be attributed to the Cr—C distances, i.e., to the second octahedral coordination sphere. As two and then four NCS ions are introduced into the coordination sphere, this preliminary peak is... 

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

Divalent chromium reduces triple bonds to double bonds (trans where applicable) [195], enediones to diones [196], epoxides to alkenes [192] and aromatic nitroso, nitro and azoxy compounds to amines [190], deoxygenates amine oxides [191], and replaces halogens by hydrogen [197,198],... 

Divalent chromium compounds (Cr ) (chromous compounds) including chromous chloride (CrCb) and chromous sulfate (CrS04). 

Unlike nickel, chromium metal does not produce allergic contact dermatitis. Some patients exhibit positive patch tests to divalent chromium compounds, but these compounds are considerably less potent as sensitizers than hexavalent chromium compounds. A case of chromium (chromic) sulfate-induced asthma in a plating worker, confirmed by specific challenge testing and the presence of IgE antibodies, has been reported. ... 

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

An organometallic dioxygen adduct of an even more fleeting kind was observed in the reaction of the divalent chromium phenyl complex Tp > Cr-... 

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