Chromium tetravalent

Westheimer has also reviewed the induced oxidations by the Cr(VI)-As(III) couple of iodide, bromide and manganous ions vide supra). The induction factor of 0.5 for Mn(II) implies an intermediate tetravalent chromium species however, the factor of 2 for iodide points to a pentavalent chromium intermediate. Both... 

In studies of the concentrations of arsenic, bromine, chromium, copper, mercury, lead and zinc in south-eastern Lake Michigan, it was shown that these elements concentrated near the sediment water interface of the fine-grained sediments. The concentration of these elements was related to the amount of organic carbon present in the sediments (161). However, it was not possible to correlate the concentration of boron, berylium, copper, lanthanum, nickel, scandium and vanadium with organic carbon levels. The difficulty in predicting the behaviour of cations in freshwater is exemplified in this study for there is no apparent reason immediately obvious why chromium and copper on the one hand and cobalt and nickel on the other exhibit such variations. However, it must be presumed that lanthanium might typify the behaviour of the trivalent actinides and tetravalent plutonium. 

All three elements form complex ammino-derivatives. Those of osmium have been very little investigated those of iridium are analogous to the anunino-derivatives of platinum on the one hand and to the ammincs of cobalt and chromium on the other whilst the platinum derivatives resemble those of cobalt, save that the metal in the platinic derivatives is tetravalent and not trivalent as in the cobalt-ammines. 

In these the metal is divalent, tetravalent, and trivalent respectively. The ammino-iridous and the ammino-iridic salts correspond to the ammino-derivatives of palladium and platinum, whilst those of the sesqui-salts are analogous to the ammino-derivatives of cobalt, chromium, and rhodium. 

Platinum forms both platinous and platinie salts, in which the metal is divalent and tetravalent respectively. Both series of salts are capable of uniting with ammonia, forming complex ammines. The co-ordination number in the platinous series is four and in the platinie series six. The latter series correspond in many respects to the chromic and cobaltic ammino-salts, but as the metal is tetravalent, the maximum number of radicles outside the complex is four instead of three. Also, the ammino-bases from which the salts are derived are much more stable than those of chromium or cobalt. 

For vanadium and chromium the first ionization energies are much lower than the first ionization energies of phosphorus and sulphur, respectively. This explains the high heats of formation of VC13 and CrCl3. In uranium, the tetravalent state is more stable than that in tungsten because uranium as an actinide has a different electron configuration. 

Chromium rutile pigments with extremely high resistance to flocculation are obtained by applying a double-layer coating of oxides or oxide hydrates formed from tetravalent metals and oxides or oxide hydrates of aluminum [3.94],... 

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. 

Schreiber, H. D. (1977) On the nature of synthetic blue diopside crystals The stabilization of tetravalent chromium. Arner. Mineral., 62,522-7. 

Chromium(IV) dioxide is a tetravalent chromium compound with limited industrial application. It is used to make magnetic tape, as a catalyst in chemical reactions, and in ceramics (Hartford 1979). Because of its limited industrial uses, the potential for human exposure is less for chromium dioxide than for the more industrially important chromium(VI) and chromium(ni) compounds. A single chronic inhalation study in rats exposed to 15.5 mg chromium(IV)/m3 as chromium dioxide reported no respiratory, cardiovascular, gastrointestinal, hematological, hepatic, renal, or dermal/ocular effects (Lee et al. 1989). 

The most common oxidation states of chromium in inorganic componnds are 11, III, and VI. By comparison, there are few stable compounds of tetravalent or pentavalent chrominm these oxidation states are largely represented by reactive intermediates in redox reactions. 7r-Acid ligands like CO (see Tt -A del Ligand) stabilize chromium in very low formal oxidation states (down to -IV ). Table 3 lists oxidations states and coordination environments for representative chromium compounds. 

It is thought by some that the metal acts as a tetravalent element in chromium dioxide, CrOa, but this compound may also be considered as a basic chromic chromate, CrgOg-CrOg. Chromium appears to function as a pentavalent element in the oxychloride, CrOClg, and its derivatives. The perchromic adds and perchromates have Jong been thought to contain heptavalent chromium, but it would appear that... 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

Another tetravalent compound, tetrakis-(neopentyl)chromium, behaved similarly [295]. It had no activity by itself, but did adsorb on various carriers to yield active catalysts. It reacted slowly with all carriers. Even in contact with fluorided silica-coated alumina, it had to be heated to about 65 °C to start its reaction with the support. Then, gradually, the purple solution adsorbed on the carrier to yield a brown catalyst. Again, the polymerization activity paralleled the reactivity with the support, and the fluorided silica-coated alumina provided catalysts with unusually high polymerization activities. 

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. 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

An electron-valence scheme of bonds in CrSi2 is proposed, based on the assumption that the tetravalent chromium atoms are bonded to the four nearest silicon atoms, lying in adjacent hexagonal layers, by somewhat distorted d s tetrahedral covalent bonds, and the silicon atoms are bonded by sp hybrid bonds to two chromium atoms and two silicon atoms. 

In an ideal 5-V spinel cathode material, the manganese ion is tetravalent and the redox species are foreign metal ions. Nickel, copper, iron, cobalt, and chromium are known as a foreign metal The highest 5-V capacity is obtained for the composition of LiM(, Mn and LiMMnO, where M is divalent and divalent, respectively. Although a capacity of 145-147 mAh/g can be expected for the divalent metal (Ni, Cu) under the two-electron transfer mechanism, only LiNij MUj... 

---