Chromium aqua ions

None of the Cr(III) products from Equations 6 or 7 are effective crosslinkers since a chromic aqua ion must be hydrolyzed first to form olated Cr to become reactive. Colloidal and solid chromium hydroxides react very slowly with ligands. In many gelation studies, this critical condition was not controlled. Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). 

The NMRD profile of chromium(III) aqua ion (Fig. 18) is characterized by slow exchanging water protons, as clearly shown by the fact that the solvent proton relaxivity at low fields increases with increasing the temperature. The occurrence of slow exchange hinders any increase in relaxivity below 300 K, thus explaining the fact that the contact dispersion disappears in the low temperature profiles, whereas it is well shown in the high temperature profiles, as already discussed in Section I.C.8. 

The presence of second-sphere water molecules could be considered also for other metal aqua ions, like iron(III) and oxovanadium(IV) aqua ions, where the reorientational time is found to be longer than expected. However, in the other cases increases much less than for the chromium(III) aqua ion, thus suggesting that second-sphere water molecules are more labile, their lifetime being of the order of the reorientational time. 

What structure can ions of chromium(lll) have in an aqueous solution depending on the conditions Explain the structure of aqua-complex chromium(III) ions from the viewpoint of the valence bond method. 

However, it is a quite striking fourth parameter that oxidizing aqua ions under equal circumstances are more acidic (have lower pK). Thus, iron(III) is distinctly more acidic than aluminium(III) and chromium(III), copper(II) more acidic than nickel(II) and zinc(II), and quite excessive acidity is observed for mercury(II)41), palladium(II)42,81) and thallium(III) aqua ions, compared with the much smaller beryllium(II) and aluminium(III). This tendency takes extreme proportions 841 in gold(III) complexes. 

Complex reducing agents are V(ll), Cr(Il) and Fe(ll). The aqua ions of iron(II) and chromium(Il) with and t2g Cg configurations respectively (high spin d and high spin d ) are kinetically labile undergoing rapid water exchange... 

Rotzinger, ER, Stiinzi, H., and Marty, W. (1986) Early stages of the hydrolysis of chromium(III) in aqueous solutions. 3. Kinetics of dimerisation of the deproto-nated aqua ion. Inorg. Chem., 25, 489-495. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Fig. 10. A possible structure of the trinuclear aqua chromium(III) species Cr, (OH)45, . The metal ions and hydroxo bridges are indicated by O and , respectively. Another very plausible structure is 4a in Fig. 1.

Hydrolysis of ammonia or amines is often observed, but only in a few cases have such reactions proved to be useful synthetically. Base hydrolysis (aqueous NH3) of the so-called rhodo ion, (NH3)5Cr(OH)-Cr(NH3)55 +, yields the so-called cis hydroxo erythro ion, cis-(NH3)5-Cr(OH)Cr(NH3)4(OH)4+, and both this ion and its corresponding acid form, cis aqua erythro have been isolated as salts (227, 252, 253). The hydrolysis is complete within minutes, and unlike the hydrolysis of many other ammine chromium(III) complexes, is quite a clean reaction, at least in solutions of moderate alkalinity (225). The corresponding trans aqua isomer has been prepared by heating the solid... 

From the values given in Table XIII it is noted that Kd is apparently much more sensitive to variation of the nonbridging ligands than to variation of the metal ion. It is seen that Kd for all the aqua metal ions lies within a relatively narrow range of about 103 5 M-1. In contrast, chromium(III) and cobalt(III) amine complexes have Kd values which vary by at least five orders of magnitude. 

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