Chromium hydroxo

All the examples so far have been homonuclear. Complexes (14) provide examples of hetero-nuclear di- -OH cations, with M = Cu, Mn, Co, Ni or Zn, in solution.73 NiI+/Al3+ dimers with di- -OH have been characterized in the solid state, and may well also exist in solution.74 Although cocrystallization of cobalt and chromium hydroxo complexes normally gives the statistically expected mixture of Co2, Cr2 and CoCr di-/i-OH species (difficult to separate), if optically active starting materials are used a very high yield of the mixed dinuclear species [(en)2Cr(jt-OH)2Co(en)2]4+ can be obtained. 

Chromium(III) hydroxide dissolves in acids to form hydrated chromium(lll) salts in concentrated alkali, hydroxo-complexes [Cr(OH)g] are formed. 

Free, ionic species of metals are at their highest concentrations at lower pH, so metals tend to be more bioavailable under these conditions.121128 At acidic pH, more protons are available to saturate metal-binding sites.99 For example, metals are less likely to form insoluble precipitates with phosphates when the pH of the system is lowered because much of the phosphate has been protonated. Under basic conditions, metal ions can replace protons to form other species, such as hydroxo-metal complexes. Some of the hydroxo-metal complexes are soluble, such as those formed with cadmium, nickel, and zinc, whereas those formed with chromium and iron are insoluble. 

Table 8.5 Stability of g-Hydroxo Bridged Chromium(lII) Complexes...

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

These salts are purple in colour, and yield a purple solution when dissolved in water. They may be prepared from hydroxo-diaquo-triammino-chromie iodide, [Cr(NH3)3(H20)2(H0)]I2.H20,2 or from chromium tetroxo-triammine, Cr04.8NH3.3... 

The corresponding dipyridino-salts, [Cr py2(H20)2R2]R, are prepared from hydroxo-diaeido-aquo-dipyridino-chromium, [Cr py2H20 (OH)R2J, by treatment with acids. They are green in colour, and crystallise in green needles or leaflets.5... 

Tetraethylenediamino-diol-dichromic Salts, [Cr2en4(OH)2]R4. —When cis-hydroxo-aquo-diethylenediamino-chromic salts or the hydroxo-bisaquo-salts are heated they lose water, and form diol-chromium salts according to the equation... 

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 ... 

The easy formation of hydroxo- or oxo-bridged Cr111 polymers in basic aqueous solution, the comparative lability of the Cr—N bond, and the precautions needed to obtain chromium(II) complexes compared with cobalt(II) complexes have meant that the preparative chemistry of chromium(III) is more difficult than that of cobalt(III). A greater variety of non-aqueous solvents is now in use, and there is greater knowledge of chromium(II) chemistry to be exploited in the preparation of chromium(III) complexes generally, but few new methods of preparation of amine complexes have been devised since the early work. 

HYDROXO-BRIDGED COMPLEXES OF CHROMIUM(III), COBALT(III), RHODIUM(III), AND IRIDIUM(III)... 

In this section the methods which have been used to gain structural information are briefly summarized. The term structure is used in this context in its broadest sense, including more qualitative observations concerning the skeleton of the bridging atoms. As a general rule, the hydroxo-bridged polynuclear complexes of chromium(III) and cobalt(III) can be isolated as well-defined crystalline salts and it is therefore quite natural that single-crystal X-ray structure analysis has... 

Fig. 1. The di-, tri-, and tetranuclear structures observed in X-ray crystal structures of hydroxo-bridged oligomers of cobalt(III), rhodium(III), iridium(III), or chromium(III) structures 4b, 7b, and 7c have never been observed, but the last two have been mentioned as possible structures for two of the known isomers of Cj4(OH)66+...

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

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... 

The cis/trans isomerization reaction, Eq. (24), has been applied in the preparation of salts of the cis isomers of the chromium(III) complexes with L3 = (NH3)3 or tacn (319). For these species Eq. (24) equilibrium is shifted to the right, while the corresponding equilibria with the diaqua or dihydroxo species, respectively, are shifted to the left (Table X). The increased stability of the cis aqua hydroxo species can be explained in terms of intramolecular hydrogen bond formations (Section VI,C). As mentioned above, the corresponding cobalt(III) and rhodium(III) complexes have been isolated as salts only in the case of the trans-(H20)L3M(0H)2ML3(H20)4+ cations, but it seems very probable that their cis isomers could be prepared by reaction Eq. (24). 

Nearly all the reported studies have been on chromium(III) complexes (cf. Table XVI). The chromium (III) complexes L4Cr(0H)2CrL44+ with ammonia or aliphatic diamines (en and tn) deprotonate in strongly basic solutions to form blue /i-hydroxo-/i-oxo species, which in some cases have been isolated as stable and crystalline salts, e.g., A,A-... 

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