Hydroxo polynuclear

M H20)sf (M = Al, Cr, Fe, Co), so the existence of a chromophore [M(0)6] in every heteropoly anion was claimed. The absorption spectrum was measured for 2 (NH4)20 C02O3 10 M0O3 12 H2O, too since the spectnxm exhibited an absorption band characteristic of a p-hydroxo polynuclear complex in addition to d-d absorption bands, the compound was considered to have a kind of polynuclear structure. 

The hydrolysis of metallie ions often leads to hydroxo polynuclear complexes. In water, for example, Fe(ni) gives rise to the following equilibria (simplified writing) ... 

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

Figure l.4 The structural variety of trinuclear ferric hydroxo complexes, Fe3(OH)f i, that could form from Fe2(OH)24+ at low pH. Iron atoms are indicated as full circles, OH as open circles. The arrow indicates a transition from dinuclears to trinuclears (p = 2 to p = 3), which is realistic as an early step in the growth of polynuclears. (From Crichton, 1991.)... 

However, ageing of these virtually monodisperse systems was shown to promote an increase in width in favour of length. There was no change from hydroxo to oxo bridges, but instead there was an alteration in polynuclear shape. While the internal structure was maintained, it was found that the polynuclears became shorter and wider, as the mononuclear ferric species released from the ends of the needles by acid cleavage redeposited on the centres of the molecules. 

The polynuclear hydroxo-anions isolated as solids will also be present in solution. Their existence points to a chemistry that is formally comparable to polynuclear oxy-anion chemistry—compare [Be2(OH)7]3 to Cr20 . In both cases the protonation of a divalent mononuclear anion results in the formation of a binuclear bridged structure. 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

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. 

Since A1(N03)3 or its salt hydrates dissociates to A13+ and NOs ions in the aqueous solution, its reactions in solutions are those of Al. It is partially hydrolyzed, producing H30" and thus accounting for the acidity of its solution in water. The products constitute a complex mixture of mono- and polynuclear hydroxo species. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

At present, there are not sufficient data available for the change of enthalpy by formation of hydroxo complexes which are further complicated by precipitation and polynuclear behaviour. However, some of Julius Thomsen s calorimetric measmrements seem to indicate that heat frequently is evolved with hard central atoms in contrast to formation of the corresponding fluoro complexes. On the other hand, sulphate and carboxylates behave like F according to Ahrland. 

The bulk of the systematic work has been carried out so far in this area. Even then most of it refers to the first period, and data for the second and third periods are needed. The predominance of hydroxo and polynuclear cationic species with this latter group will undoubtedly complicate the measurements. 

Aluminum nitride UFPs have been synthesized by thermal decomposition from many kinds of precursor such as polyminoalanef l/ ) AIH(NR)] (50), aluminum polynuclear complexes of basic aluminum chloride (BAC) or basic aluminum lactate (BAL) (51), and (hydroxo)(succinato) aluminum(lll) complex, A1(0H)(C4H404) jfLO (52). These precursors were calcined under N2 or NH, gas flow. The calcination temperatures, which depend on the individual precursor, can be lower by 600-200°C than the 1700°C in ihe conventional carbothermal reduction method. The XRD measurements at intermediate stages of the calcination process showed the phase change from an amorphous state to a trace of y-alumina with very fine grains and finally to wurtzite-type AIN (51,52). Lowering the calcination... 

Studies on bidentate and higher dentate amine ligands have indicated that complex formation with silver(I) was rather more complicated than for monodentate amines. Polynuclear, protonated and even hydroxo complexes have been postulated to occur in solution. 

Recently the stability of silver(I) complexes of iV-methyl-substituted 4-methyldiethylenetriamines has been investigated by potentiometric pH and pAg measurements. Besides mononuclear complexes, polynuclear and protonated complexes were formed. Evidence of hydroxo complexes was also presented.48... 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

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... 

Hydrolysis is strong acid in which all hydroxo bridges are cleaved, followed by identification of the various mononuclear species and a determination of their molar ratios, may provide extremely valuable information. A straightforward example is the cleavage of the tetra-nuclear species Cr4(NH3)12(OH)66+, which yields Cr(H20)63+ and cis-Cr(NH3)4(H20)23+ in a ratio of 1 3 (40). Since it could be demonstrated at the same time that the polynuclear cation does not exhibit acid base properties in the pH region for terminally coordinated water, it was concluded that the only possible structure was 6 in Fig. 1, as later confirmed by a crystal-structure analysis (41). 

Formation of hydroxo-bridged complexes by hydrolysis in aqueous solution is, not surprisingly, the most common preparative method. As a rule, such reactions give quite complex product mixtures containing species with different nuclearities, each of which may be present in many isomeric forms. The fact that most of the preparative procedures employed lead to the isolation of one single and pure isomer probably more often reflects favorable solubility properties rather than stereospecificity. In some cases ion-exchange chromatography has been used to isolate the polynuclear species, but systematic analysis of hydrolysis mixtures by this technique has been reported for only a few systems. 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

---