Mononuclear hydroxo complexes

There are as yet no structural data on the mononuclear hydroxo complexes formed at high pH. Schmidbaur et al. (91) have obtained the structure of a compound that had been crystallized from a solution at pH 13.2. The compound contains the anion [Be4(OH)10]2, which has a structure resembling that of adamantane or P4O10, as shown in Fig. 9. 

Gallium hydroxide is amphoteric, and is a much stronger acid than aluminum hydroxide. For Ga(OH)3 the first acid dissociation constant is 1.4 x 10-7 [for Al(OH)3 the value is 2 x 10-11].1 Polymerization occurs in aqueous Ga3+ solutions to which OH- is added,533 but this tendency is less than in the case of aluminum solutions (Section 25.1.5.1). The formation constants of mononuclear hydroxo complexes of Ga, including Ga(OH)4, and the hydrolysis constants of gallium ions have been measured by a competing ligand technique.534... 

In the case of the purely aliphatic ligand 2,2,6,6-tetrakis(amino-methyl)-4-azaheptane (12), complex formation with copper appears to proceed in two steps, as elucidated by titration experiments with the fully protonated ligand (12 5 HC1). Three and two protons from (Hr,12) + are sequentially abstracted, and the predominant species after full deprotonation appears to be a dinuclear complex in which two copper(II) ions are coordinated, each in square planar fashion, by the l,3-diaminoprop-2-yl units of two molecules of pentaamine ligand, thus forming a macrocyclic complex of composition [Cu2(12)2]4+ (23). The UV/vis spectral data show an interesting solvent dependence, suggesting an equilibrium between [Cu2(12)2]4 + and two equivalents of mononuclear complex [Cu(12)]2+ under suitable conditions. ESR spectroscopic data are also compatible with the formulation of a dinuclear species. Further addition of base to an aqueous solution of [Cu2(12)2]4+ gives the mononuclear hydroxo complex [(12)Cu(OH)]+, as inferred from the UV/vis spectroscopic data. 

The Al3+ ion shows a propensity to suffer hydrolysis which can produce the formation of various mononuclear hydroxo complexes. At pH around 7.0 it often precipitates as Al(OH)3, which redissolves leading to the formation of aluminate, Al(OH)4. After this, polynuclear complexes such as Al2(OH)2, Al3(OH)4] and Al13(OH)32 can also form. These species can produce, in a very slow process, oligomeric complexes, which are necessary precursors of the macromolecular polymer [Al(OH)3]n [16, 17]. In Fig. 2 we can observe the distribution of the Al3+ aquo ion and the species for 100 mM and 10 pM total aluminum. The maximum concentration of Al(OH)3 is reached at about neutral pH. At pH > 7.0 the predominant species is aluminate ion A OH). ... 

Chemisorption is very pronouneed if the radionuelide is able to react with the surface. An example is the sorption of hydroxo complexes of tri- and tetravalent elements by the silanol groups on glass surfaces. Sorption increases with the formation of mononuclear hydroxo complexes in solution and deereases with the eondensation to polynuclear complexes at higher pH values. Several measures may be taken to suppress ion exchange or chemisorption of traces of radionuclides on glass surfaees ... 

The competing ligand system alizarin-3-sulphonic acid-OH" has been used to determine formation constants of mononuclear hydroxo-complexes of Ga at 25 °C, and the hydrolysis constants of gallium ions at I = 0.1—l.O.519... 

Formation constants of mononuclear hydroxo-complexes of aluminium were determined spectrophotometrically at 15 C using alizarin-3-sulphonic acid as a competing ligand. The hydrolysis constants of A1 ions at ionic strengths in the range 0.1—1.0 were also calculated.The results for Al(OH) " are shown in Table 10. 

Table 10 Formation and hydrolysis constants of mononuclear hydroxo-complexes of aluminium (in)...

DAV/TOR2] Davidov, Yu. P., Toropov, I. G., The hydrolysis of Th" cations to give mononuclear hydroxo-complexes in solution, Russ. J. Inorg. Chem., 31, (1986), 198-201. Cited on pages 156, 157, 502, 592, 593. 

TABLE 5-1 Equilibrium Constants for Mononuclear Hydroxo Complexes... 

Oxidative addition of water to iridium(I) has also been known [185], In this reaction, reaction of [Ir(PMe3)4]+[PF6] with water in THF gave a rare mononuclear hydroxo complex c/5-[Ir(H)(OH)(PMe3)4] [PF6] in high yield. A neutral iridium(I) complex IrCl(dmso)3 is recently reported to show high activity toward oxidative addition of water [186],... 

Table 25.1 Stability constants of some mononuclear hydroxo complexes...

Table 25.2 Formation constants of mononuclear hydroxo complexes of some metallic ions from the corresponding insoluble oxides or hydroxides...

Quite evidently, this nomenclature is generalized to other complexes (see Chap. 35). Table 25.2 mentions formation constant values of mononuclear hydroxo complexes, the complexes being formed from the corresponding insoluble oxides or hydroxides. These constants permit us to calculate the concentrations of the formed complexes when there is a precipitation. They also permit us to calculate the number of moles that are precipitated. 

Davydov, Y.P. and Toropov, LG. (1986) Hydrolysis of Th cations with formation of mononuclear hydroxo complexes in solution. Zh. Neorg. Khim., 31, 351-355. 

Gordienko, V.I. (1973) pH-potentiometric determination of the composition of hydroxo complexes and of the hydrolysis constants of salts of weak bases at high ionic strengths. I. Simple mononuclear hydroxo complexes. Zh. Obshch. Khim., 43, 2109-2113 (English translation). 

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