Chromium aquo complexes

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

The possibility of insertion of aquo- and amine-copper(II) complexes or chromium(III) complexes into a PMS matrix during the synthesis was discussed [40,41]. Tetraamino complexes of copper such as [Cuen2(H20)2] and hexa- and penta-amino complexes of chromium [Cren3] + and [Cr(NEt3)5Cl] were used. The synthesis of the adsorbents was performed in aqueous solutions at pH 9-10, in which amino complexes of transition metals undergo alkaline hydrolysis forming hydroxo-complexes. The presence of hydroxyl groups in the coordination sphere of the metal allowed a polycondensation reaction between the complex and hydrolysis products of sodium methylsiliconate (see Scheme 7). 

In its reversed mode, FIA is fully adaptable to Industrial process control [49-51], and to studies of the speciation of various elements In waters [102]. One of the most promising developments in the latter area is the speciation of chromium by means of the configuration depicted in Fig. 6.20. This uses a combined giass-calomel microelectrode incorprorated in the sample stream prior to the simultaneous injection of the reagents (Ce and 1,5-diphenylcarbazide for Cr3+ and Cr(VI). The data obtained for the concentration of these two species, together with the sample pH and the constants corresponding to the equilibria in which both oxidation states are involved, allow the calculation of the concentration of up to nine different chromium species aquo complexes and hydroxylated forms of Cr(III) and ionic, molecular and dimeric forms of Cr(VI) [103]. 

An rFIA-asynchronous merging zones configuration has be used for the speciation of up to nine different chromium forms —aquo complex, mono-, di- and tetrahydroxylated Cr(III) and molecular, anionic and dimeric Cr(VI). It Includes a glass-calomel microelectrode Inserted In the sample stream prior to the merging with the reagents, and a microcomputer which acquires the measured pH and chromium concentrations —Cr(VI) and total Cr. These data are pro-cesssed by a computation program In which the equilibrium constants of the... 

Oxo anions are known to catalyze loss of ligands from aquo-chromium(III) complexes as discussed in Section 5.2.2.1.2. Two groups have studied the reaction of Cr(III) with xylenol orange, in the presence " and absence " of... 

Whereas the great majority of aquo-chromium(m) complexes are inert to formation reactions, the tetra-(p-sulphonatophenyl)porphine complex is remarkably labile—stopped-flow techniques are required to monitor replacement of the co-ordinated waters of this complex by, for instance, chloride, cyanide, or pyridine. Analogous cobalt(in) complexes are also substitution labile. This has been attributed to the possibility of a redox mechanism for substitution. In view of the difficulty of reducing chro-mium(ni) to chromium(n) such a mechanism seems unlikely here, so that the porphine ligand may be promoting reactivity to substitution by electronic effects. ... 

This is a common form of rate law for aquations of aquo-chromium(ra) complexes. Activation enthalpies are greater than 30kcalmol- for both aquation routes for this 3-picoline complex. ... 

Many reported rate laws for aquation of aquo-chromium(m) complexes do not contain the A i[H+] term. This will generally be the case if the pH range covered by the experiments is entirely in the fairly to strongly acid region the p fiTa of [Cr(OH2)6] + itself is 3.82 (lef. 196). ... 

The kinetics of formation of oxalato-complexes both from [Cr(OH2)s] + (refs. 313, 314) and from mixed ligand-aquo-chromium(ni) complexes have been investigated. Some of the results of a detailed study of the kinetics of formation of the mono-, bis-, and tris-oxalato-complexes from [Cr(OH2)6] +, as a function of oxalate concentration, pH, temperature, and ionic strength, are given in Table 18. Second-order rate constants for each step in this... 

The Cr(VI) ion is present in water at pH >7 as the oxo form [Cr04] . At pH < 7. protonation is possible because the polarization of oxygen by the Cr(Vl) ion is weaker than by the Mn(Vll) ion. [CrOjfOH)] exists in solution around pH 4. In a more acidic medium, the Cr02(0H)2 form predominates in very dilute solution. However, it is impossible to obtain cationic forms of Cr(VI) corresponding to an additional protonation of the oxygen bound to chromium. This occurs with Cr(lll) which forms the [Cr(OH2)6j aquo complex at pH < 2. Around pH 4-5, the liydroxo form Cr(OH)3(OH2)3 leads to the hydroxide which redissolves in the alkaline medium into [Cr(OH)4(OH2)2], but the oxo ligand does not appear in... 

Hexa-aquo complexes of trivalcnt elements are stable in an acidic medium at room temperature. Hydroxylation of these complexes by water may, however, take place by heat treatment (see Section 1.4). Heating of an acid solution of Al(III) ions to about 80-100 °C cau.ses the formation of boehmite 7-AIOOH [67,68]. In similar conditions, Fe(IIl) forms hematite. Thermolysis of acid solutions of chromium causes the formation of oxyhydroxidcs of poorly defined structure, as well as... 

Iron polycations are not as well known as chromium or aluminum polycations because of the lability of ferric complexes. Only a few polycations (dimers, trimers) have been characterized in acidic solutions (pH < 1.5) [39]. [Fe2(OH)2] and [Fc20] dimers are present in organic complexes such as L3(H20)Fe(OH>2-Fc(OH2)L3 and LjFeOFeLs, where the L3 ligand is a tridentate picolinate and L5 a tridentate amine [16,40,41]. Other polydentate ligands, such as proteins, are able to stabilize many polynuclear iron complexes [42-45]. The existence of lire aquo complexes [(H20)4Fe2(0H)2(0H2)4] " and [(H20)5Fe20(0H2)s] is very probable in spite of the lack of structural data. 

Chromium(lII) differs from aluminum and iron in its exceptional chemical inertia. The fast addition of a base in a solution of Cr + ions also causes the formation of a gel owing to cancellation of the charge on the aquo complexes or polycations. Various hydrated hydroxides ate formed, and they are made of aggregates of species that previously existed in solution. 

This is by far the most stable and best-known oxidation state for chromium and is characterized by thousands of compounds, most of them prepared from aqueous solutions. By contrast, unless stabilized by M-M bonding, molybdenum(III) compounds are sparse and hardly any are known for tungsten(III). Thus Mo, but not W, has an aquo ion [Mo(H20)g] +, which gives rise to complexes [MoXg] " (X = F, Cl, Br, NCS). Direct action of acetylacetone on the hexachloromolybdate(III) ion produces the sublimable (Mo(acac)3] which, however, unlike its chromium analogue, is oxidized by air to Mo products. A black cyanide,... 

These hydroxo-salts are all sulphur-yellow crystalline substances. The acid residues are hydrolysable and hence outside the co-ordination complex, and the aqueous solutions, unlike the hydroxo-salts of chromium-and cobalt-ammines, are neutral to litmus, a fact which Werner suggests is due to the smaller tendency of the hydroxo-radicle attached to ruthenium to combine with hydrogen ions. This tendency is much less than in the case of the ammines of cobalt and chromium, but that it still exists is indicated by the increased solubility of these hydroxo-compounds in water acidified with mineral acids, and from such solutions aquo-nitroso-tetrammino-ruthenium salts are obtained thus ... 

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. 

Other aquo-cobalt(III) complexes, such as Co(en)2(H20)2 3, react with nitrite in the same way and for the reaction of the chromium complex, Cr(NH3)5H20+3 with nitrite, this seems to be the predominant, if not the exclusive, mode of substitution. 

The equilibrium constant of reaction (1), K = [Cu ][Cu ]/[Cu ], is of the order of 10 thus, only vanishingly small concentrations of aquo-copper(I) species can exist at equilibrium. However, in the absence of catalysts for the disproportionation—such as glass surfaces, mercury, red copper(I) oxide (7), or alkali (311)—equilibrium is only slowly attained. Metastable solutions of aquocopper(I) complexes may be generated by reducing copper(II) salts with europium(II) (113), chromium(II), vanadium(II) (113, 274), or tin(II) chloride in acid solution (264). The employment of chromium(II) as reducing agent is best (113), since in most other cases further reduction to copper metal is competitive with the initial reduction (274). 

There are various other cases, especially the [Cr(H20)5X]2 +-[Cr(H20)6]2+ exchanges, in which retention of the X groups shows that they must be bridges in the activated complex. Also, when Fe3 + is reduced by Cr2+ in the presence of halide ions, the chromium(m) is produced as [Cr(H20)5Xl2+. Similar phenomena are found also in the Coll Co111 aquo system and fairly generally in Pt"-Ptlv systems. 

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