Chromium cation ordering

The rate constants of this bimolecular reaction for a given organo-chromium cation change in order of the driving force, with fis increasing in the series Ig- < SCNg- < Br. ... 

Local description of the arrangement around cations concludes unambiguously for [Cu-Cr-Cl] in a cationic ordering. The evidence of similar ordering for [Zn-Cr-Cl] was only obtained by a combined EXAFS and UV-Vis study of the formation of this LDH in solution [18], The structural pathway so-reported involves the heterocondensation between hexa-aquo zinc(II) complexes and deprotonated chromium monomers. 

Oxyfluoroniobates, M2Nb05F, containing trivalent metals (where M = Ti, V, Cr) have the same type of structure [264], except for Cr2Nb05F, which has a tri-rutile type structure. This exception is related to the ordered, rather than statistical, distribution of chromium and niobium cations in the oxyfluoride octahedrons, which leads to a corresponding increase in cell parameter c. 

In the presence of certain cations [sodium, potassium, lithium, calcium, aluminium, chromium, and iron(III)], co-precipitation of the sulphates of these metals occurs, and the results will accordingly be low. This error cannot be entirely avoided except by the removal of the interfering ions. Aluminium, chromium, and iron may be removed by precipitation, and the influence of the other ions, if present, is reduced by considerably diluting the solution and by digesting the precipitate (Section 11.5). It must be pointed out that the general method of re-precipitation, in order to obtain a purer precipitate, cannot be employed, because no simple solvent (other than concentrated sulphuric acid) is available in which the precipitate may be easily dissolved. 

Kinetic studies of the oxidation of sulphoxides to sulphones by chromium(VI) species have been carried out131-133. The reaction has been found to be first order with respect to the chromium(VI) species and the sulphoxide and second order with respect to acid. At high sulphoxide concentrations the order with respect to sulphoxide is two. The proposed mechanism involves an electron transfer from the sulphoxide to the active chromium(VI) species (HCr03+ in strong acidic media) in the rate-determining step producing a sulphoxide radical cation which further reacts to give the sulphone. 

A neutral chromium atom has 24 electrons, so the corresponding Cr cation has 21 electrons. The first 18 electrons follow the usual filling order to give the argon core configuration ... 

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. 

Comparing the 12-6 parameters, the largest differences with previously reported parameters are obtained for the bivalent and trivalent cations. The case of chromium is particularly interesting, where the size parameter is reduced to half, and the energy parameter increases several orders of magnitude. 

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