Divalent Chromium Compounds

The most stable state of chromium is the +3 state compounds of hexavalent chromium are almost as good oxidizing agents as elemental chlorine, whereas compounds of Cr(II) ( chromous compounds) are potentiometrically more easily oxidized than cadmium metal. Divalent chromium, like Ag(II) and Au(III), may exist in equilibrium with aqueous media only as the cation of a relatively insoluble salt or in a slightly dissociated complex. However, solutions containing the blue Cr24 ion may be 

The brick-red acetate, Cr(OAc)2, is one of the few insoluble acetates known. Of some interest are the iodide, Crl2, and the sulfide, CrS these are both prepared from combination of the appropriate elements at high temperatures and are thus formed in preference to the corresponding compounds of Cr+3. 

A number of the intensely colored hydrated salts of trivalent chromium (the nitrate, sulfate, chloride, and the alum ) are doubtless familiar. The octahedral hexaaquochromium(III) ion, Cr(H20) 3, is violet, but aqueous solutions of chromic salts are often green as a result of replacement of water molecules in the complex by the anions present. The change of color occurring on heating a solution containing trivalent chromium and chloride ions should be recalled  

It is frequently said that the behavior of trivalent chromium resembles that of trivalent aluminum, presumably because of the similarity in radii of the tw o ions (A1+8, 0.45 A Cr+3, 0.55 A). The structures of the solid oxides, fluorides, and chlorides are analogous for the two metals, and both form salts of the alum type (for example, KA1(S04)2 12H20, KCr(SeC 4)2 12H20). Chromium(III) sulfide, (Cr2S3) like aluminum sulfide, cannot be made in aqueous systems. 

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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. ... 

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

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. 

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],... 

The important valence states of chromium are II, III, and VI. Elemental chromium, chromium(O), does not occur naturally. The divalent state (II or chromous) is relatively unstable and is readily oxidized to the trivalent (III or chromic) state. Chromium compounds are stable in the trivalent state and occur in nature in this state in ores, such as ferrochromite (FeCr204). The hexavalent (VI or chromate) is the second most stable state. However, hexavalent chromium rarely occurs naturally, but is produced from anthropogenic sources (EPA 1984a). Chromium in the hexavalent state occurs naturally in the rare mineral crocoite (PbCr04) (Hurlburt 1971). 

The double oxides are sometimes referred to as mono- and triuranates of the transition metals, even though the existence of U(VI) in some of the compounds is questionable. This is particularly true for CrU04 since divalent chromium is readily oxidized. When the double oxides are referred to as uranates in this paper, it should be borne in mind that... 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

As the inorganic additives to polyamides recommended in the literature, we should mention primarily salts of trivalent chromium [77, 78). The use of compounds of di- and hexavalent chromium is also possible. However, during the process of treatment of the polyamide with the stabilizer, divalent chromium is oxidized, while hexavalent chromium is reduced to the trivalent form. Thus, for example, polyamide fibers are treated first with potassium bichromate and then with sodium thiosulfate [77]. Certain chromium dyes also exhibit a protective action against sunlight [79]. 

The alkoxide derivatives of divalent chromium (as well as manganese, cobalt, and nickel) are all insoluble non-volatile products and their insolubility may be attributed to their polymeric nature. Chromium trialkoxides are also insoluble, except the tri-tert-butoxide which is a soluble dimeric compound. " Chromium tri-tert-butoxide when heated in vacuo yielded volatile Cr(OBu )4 and nonvolatile [Cr(OBu )2] ... 

The difficulties in synthesizing the mixed spinel seem related to both thermodynamic and kinetic issues. Hydroxides (and/or basic salts) of divalent elements are more stable than the ferrite at low temperatures. The large reactivity difference between iron and chromium explains the rapid crystallization of iron oxides or oxyhydroxides compared with the chromium compounds, as well as the segregation of both elements. 

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 ... 

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. 

Ammino-eompounds containing divalent anion are unknown, and the group is only represented by such compounds as aquo-pentabromo-chromium, [CrH2O.Br5]M2. 

Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(II) (56) is another, along with allyl-Cr(II) (52, 57). Allyl-Cr(III) is also active (52, 57-61). -Stabilized alkyls of Cr(II) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard. 

In keeping with its 4d%5s electron configuration, molybdenum forms many compounds in which its oxidation state is 6+. to an even greater extent than chromium. Also, like chromium, it forms compounds in which II is divalent and those in which it is trivalenl unlike chromium, il forms a number of pentavalenl compounds, and a few more tetravalent compounds, especially complexes. 

Stoichiometrically Simple Compounds C. DIVALENT METAL CHROMIUM (HI) OXIDES... 

Except for compounds with ir-add ligands, there is not a great deal of similarity to chromium. The divalent state, well defined for Cr, is not well known for Mo and W except in strongly M—M bonded compounds and the abundance of highly stable Crm complexes has no counterpart in Mo or W chemistry. For the heavier elements, the higher oxidation states are more common and more stable against reduction. 

The chromous salts, derived from the oxide CrO, arc analogous to the salts of divalent vanadium, manganese, and iron. This is seen in the isomorphism of the sulphates of the type R" SOj-THgO. The stability of such salts increases in the order of the atomic number of the metal. The chief basic oxide of chromium is the sesquioxidc CraO, which is closely allied to ferric oxide, and, like the latter, resembles aluminium oxide. The hydroxide, Cr(OH)3, with bases yields chromites analogous to, but less stable than, the aluminates. Chromic sulphate enters into the formation of alums. The chromic salts are very stable, but in the trivaJent condition the metal shows a marked tendency to form complex ions, both anions and cations thus it resembles iron in producing complex cyanides, whilst it also yields compounds similar to the cobaltamines. 

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