Chromium oxoanions

Chromium in the +6 state forms two different oxoanions, the yellow chromate ion, Cr042-, and the red dichromate ion, Cr2072- (Figure 20.11). The chromate ion is stable in basic or neutral solution in acid, it is converted to the dichromate ion ... 

Oxidation A half-reaction in which there is an increase in oxidation number, 88 chromium, 548 electrolysis and, 498 fluorine, 557 halogens, 557-558 oxoacids, 568-570 oxoanions, 568-570 species strength, 506-507q transition metals, 546t zinc, 86-87... 

Cr3+ and Cr6+ are the most stable oxidation states of chromium, but with the only difference that while +3 oxidation state is cationic where as the +6 oxdation state is oxoanionic. However, the other oxidation states of +1, +2, +4 and +5 are also known for chromium, especially in aqueous solution at different pH. Inter-conversion of these oxidation states too is very frequent. With this view, an attempt is made here to examine the effect of ultrasound on the inter-convertibility of chromium among various oxidation states in aqueous solutions. The details of this study is reported in the literature [36]. 

Pankaj, Chauhan (2004) Sonochemical studies of aqueous solutions of chromium and manganese in their cationic and oxoanionic states. Ind J Chem 43(A) 1206-1209... 

Another rather unusual case of rapid aquation is provided by those oxoanion ligands MO in which the M-0 bond is sufficiently weak for it to break rather than the Co-0 bond. This time we have an intramolecular reaction of a coordinated ligand. A recent example is provided by aquation of the [Co(NH3)5(OMo03)]+ cation (57). This aquation pathway is actually more commonly encountered for chromium(III) complexes. 

The polymerization of simple tetrahedral oxoanions involves (l)the comer linkage of tetrahedra, as with the chromates and some vanadates, and (2), much more commonly, especially with Mo and W, an expansion of coordination munber to six, and the edge- and comer hnkage of MOe octahedra. The ability of Mo, and to display variable (four-, five-, six-, and occasionally sevenfold) coordination by an oxide ion is responsible in part for the large numbers of polyoxoanions formed by these elements. In contrast, hexavalent chromium is effectively limited to four coordination, and Nb and Ta to six coordination by the oxide ion. 

Thus, the highest oxides or chromium(VI) and molybdenum(VI) were established to destroy molten KN03 with subsequent evolution of N02 termination of this destruction resulted in stabilization of the pO values of the melt [67], This value determined by the method of potentiometric titration was found to reduce as the temperature of the melt was elevated considerably. Such a behaviour of the acidic solution in the nitrate melt was explained by the formation of nitrogen pentoxide, N2Os, or nitronium cation, NOj, in the acidic solution, together with the corresponding oxoanions of Cr(VI) and Mo(VI). Since the added acids are very strong, their reaction with the melt... 

The cw-labilizing effect of oxoanions such as nitrate or acetate when co-ordinated to chromium(iii) was discussed in Volume 4 of this series. This is believed to occur through the formation of transient bidentate species through chelation of the oxoanion. From studies of the nitrate substitution reactions of [Cr(H20)50N02] + in aqueous DMSO, the rate constant at 298.2 K for water exchange with [Cr(HaO)5 ONOa] " is reported to be ca. 8 x 10 s , which is several orders of magnitude larger than for [Cr(H20)e] +, The rate constant for the aquation of [Cr(HaO)6-ONOa] + is 10 A = 8.8 s i at 298.2 K. 

A number of oxoanions are known to react rapidly with co-ordinated water molecules in cobalt(in), chromium(iii), and other inert metal complexes, reactions which occur without cleavage of the metal-water oxygen bonds. Some recent examples have appeared. Relaxation studies have been used to study the rapid... 

Recently c/ -activation has been recognized as playing an important part in chromium(m) substitutions whenever potentially chelating co-ordinated oxoanions are present e.g. NOs , SOa, AcO, S04 ). This makes it important that the correct counterion is chosen in studies of substitution reactions of cationic complexes. For example, replacing nitrate ion by perchlorate ion makes a significant difference to the rates of acid hydrolysis of some chromium(m) ammines. ... 

CrfNOsls chromium(lll) nitrate SnCl2 tin(ll) chloride TABLE 2.3 Oxoanions of Nitrogen, Suifur, and Chiorine... 

Oxoanions. Aquation of the four nitrito-chromium(m) complexes listed in Table 9, in acid solution (pH< 1), follows the rate law... 

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