Hydroxy complexes

Hydroxide Fe(OH)3 (Fe + plus OH ) has definite existence and there are many ill-deiined hydrates used as pigments. FeOOH has two forms goethite and lepidocrocite. Colloidal Fe(OH)3 is easily obtained as a deep red sol. Many Fe(III) hydroxy complexes are known. Fe(OH)2 may be formed from Fe and OH" in the absence of O2 but it is very readily oxidized. 

Zinc forms salts with acids but since it is amphoteric, it also forms zincates, eg, [Zn(OH)2 H20] and Z.n([7) ). The tendency of zinc to form stable hydroxy complexes is also important because some basic zinc salts are only slightly soluble in water. Examples are 3Zn(OH)2 ZnSO [12027-98-4] and 4Zn(OH)2 ZnCl2 [11073-22-6] which may precipitate upon neutralization of acidic solutions of the salts. 

Figure 3.100 Bond lengths in platinum(IV) ammine hydroxy complexes.

Fig. 26. (a) A stereoview of the hydroxide-bridged complex and (b) the crystal structure (93) of the complex cation CudIXOHXO-BISTREN), showing the hydrogen bonding that appears to promote formation of the bridged hydroxy complex. 

In the presence of strong alkali, the rhodium analog of 62, or RhCl(C8H,2)PPh3, hydrogenates aliphatic ketones at 1 atm and 20°C, and after treatment with borohydride the systems similarly reduce aromatic ketones to the alcohols (526). Deuterium exchange data for acetone reduction were interpreted in terms of hydrogen transfer within a mononuclear hydroxy complex containing substrate bound in the enol form (63). 

They showed this process can proceed in aqueous solution by the oxidation of the hydroxy complex VO(OH)+, according to the reaction... 

An even more serious problem can arise when dissolved species expected to predominate at high temperatures are undetectable at 25°C or are only present at concentrations which are too low for them to be adequately characterized thermodynamically. Examples are certain transition metal chloro-complexes (9,10) and mixed complexes of such metals with hydroxide and another ligand (11,12). Thus it seems that chloride complexing so alters the aqueous chemistry of copper and gold that supposedly inert gold components in autoclaves are reversibly oxidized by Cull (10) and it is likely that mixed oxine and hydroxy complexes of Fell contribute considerably to the gross under-estimation (by a factor of up to 10 ) of magnetite solubility in oxine (12,14). 

The problem of measuring the thermodynamic properties of aqueous transition metal ions above 100 C has also received some attention with studies on Fe + complexing with Cl (46), Br (47) and SO - (48) up to 150°C and the formation of anionic hydroxy complexes of Pb2+ up to 300°C (49). 

Oxidation of benzyl alcohol catalysed by chloroperoxidase exhibits a very high prochiral selectivity involving only the cleavage of the pro-S C-H bond. The reaction mechanism involved the transfer of a hydrogen atom to the ferryl oxygen of the iron-oxo complex. An a-hydroxy-carbon radical and the iron-hydroxy complex P-Fe -OH form. They may lead to the hydrated benzaldehyde or stepwise with the formation of the intermediate a-hydroxy cation. 

Figure 4.19 shows the extraction of Ga(III) by acetylacetone into benzene at various concentrations of total acetylacetone, [HA]t, and constant ionic strength, using the AKUFVE technique (see section 4.15.3). By comparing with Fig. 4.3, one may guess that hydroxy complexes are formed (diagram c). 

Taylor, R.M. (1988) Proposed mechanism for the formation of soluble Si-Al and Fe(lll)-Al hydroxy complexes in soils. Geoderma 42 65-77... 

The few 3-metalla -l,2-dioxolane complexes of rhodium and iridium isolated so far have been highly reactive species. Simply by exposure to daylight they rearrange to the very unusual formylmethyl hydroxy complexes [M(/c -tpa)M(OH)(iii-CH2CHO)](X) and [Rh(/c4 dpda-Me2)(OH)(Tii-CH2CHO)] (PFe) in the solid state (Scheme 13) [84]. An alternative route to these formylmethyl hydroxy complexes is the oxidation of a 2-rhoda oxetane with hydrogen peroxide [67] (Scheme 13). 

Treatment of the /J-hydroxy complex 15 with two equivalents of strong base followed by alkylation produces a mixture of the diastereomers 20 and 21 with an anomalously low d.r.27. The low degree of diastereofacial discrimination has been rationalized by invoking the formation of both rotamers of the initially formed alkoxide, 16 and 17. Rotamer 16 undergoes a-proton abstraction by a second equivalent of base to form the chelated dianionic Tf-enolate 18 which upon alkylation affords the usual diastereomer 20. Rotamer 17 is thought to rapidly transform to a metallo-lactone species by intramolecular attack of the alkoxide upon the proximate carbon monoxide ligand, which must occur faster than conversion to the less sterically encumbered conformer 16. Subsequent deprotonation to generate dianion 19, which is constrained to exist as the unusual Z-enolate, followed by alkylation provides the other diastereomer 21, which is formed in an amount nearly equal to 20. 

First, I will comment on the existence of copper(III) in complex form. I have mentioned that a hydroxy complex or oxy complex has been produced in solid state by Klemm. We have succeeded recently in preparing the same compound, sodium... 

This will give the rate of a base catalyzed reaction involving simple complexes. Obviously if hydroxy complexes are involved, they must be used in the rate expression in addition to, or in place of, the simple complexes. If the attack of the complexes by water is important, the terms in kh can be included to obtain a term analogous in form to that resulting from the attack of hydroxide ion. One result of the temperature dependence is that gross rates of such reactions may be expected to exhibit temperature extrema. Both H and oh also show an exponential temperature dependence. 

Arnorsson, S. Andresdottir, A. 1999. The dissociation constants of Al-hydroxy complexes at 0-350 °C and Psat. In Proceedings of the Sixth International Symposium on Geochemistry of the Earth s Surface, Reykjavik, 425-428. 

The hydrated metal hydroxy complex in Eq. (1.10) is a soluble species. However, if the pH is sufficiently high, the metal hydroxide, which is relatively insoluble for most metals (apart from the alkali group metals) will precipitate. The pH value at which hydroxide precipitation occurs can be related to the acidity of the cation and is approximately equal to the pK of the cation, where the pK is minus the logarithm of the equilibrium constant of Eq. (1.10). 

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