Complex hydroxide

If sodium hydroxide is added to a solution containing zinc ion a precipitate of zinc hydroxide is formed  

This hydroxide precipitate is of course soluble in acid it is also soluble in alkali. On addition of more sodium hydroxide the precipitate goes back into solution, this process occurring at hydroxide-ion concentrations about 0.1 M to 1 M. 

To explain this phenomenon we postulate the formation of a complex ion. The complex ion that is formed is the zincate ion, Zn(OH)4, by the reaction 

The ion is closely similar to other complexes of zinc, such as Zn(H20)4 , Zn(NH3)4, and Zn(CN)4, with hydroxide ions in place of water or ammonia molecules or cyanide ions. The ion Zn(H20) (OH)3 is also formed to some extent. 

The molecular species that exist in zinc solutions of different pH values are the following  

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Derive a solubility diagram (solubility versus pH) for Zn(OH)2 that takes into account the following soluble zinc hydroxide complexes Zn(OH)+, Zn(OH)3, Zn(OH)4 . 

Determine the stoichiometry of the Pb-hydroxide complex and its formation constant. 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Porphyrin, octaethyl-, aluminum hydroxide complex cyclic voltammetry, 4, 399 <73JA5140)... 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

Scheme 6-2 Preparation of hydrido(hydroxo) and hydrido (hydroxide) complexes by oxidative addition of water...

Fig. 8. Correlation between Pearson s hardness parameter (7P) derived from gas-phase enthalpies of formation of halide compounds of Lewis acids (19), and the hardness parameter in aqueous solution (/A), derived from formation constants of fluoride and hydroxide complexes in aqueous solution (17). The Lewis acids are segregated by charge into separate correlations for monopositive ( ), dipositive (O), and tripositive ( ) cations, with a single tetrapositive ion (Zr4+, ). The /P value for Tl3+ was not reported, but the point is included in parentheses to show the relative ionicity of Tl(III) to ligand bonds. 

The complex Cu(II)2(0-BISTREN) is much more acidic than the free Cu2+ ion, by a factor of more than three log units. This is primarily due to the presence of two Cu(II) ions, because the formation constant of the Cu2(OH)+ complex is not much less than that for the Cu2(0-BISTREN) complex with hydroxide. This is not a good indication of how well two free Cu2+ ions would bind hydroxide compared to the Cu2(0-BISTREN) complex, however, since one must take into account the dilution effect operative in the chelate effect to make the comparison more realistic (90). Thus, the formation constant for the Cu2OH+ complex above applies for the standard reference state of 1 M Cu2 +. In contrast, in 10 6 M Cu2+, for example, the pH at which Cu2(OH) + would form is raised from pH 5.6 to 11.6, ignoring the fact that Cu(OH)2(s) would precipitate out long before this pH as reached. By comparison, the acidity of the Cu2(0-BISTREN) complex is not affected by dilution and would still form the hydroxide complex at pH 3.9 if present at a 10"6 M concentration. 

The magnesium hydroxide complex [TpAr2]MgOH (Ar = p-C6H4Bul) has been obtained by the reaction of the methyl derivative [TpAr2]MgMe with H20 [Eq. (29)], although it has not been confirmed that the product is monomeric, and a dimeric structure is possible (148). 

A cadmium hydroxide complex of composition [TpMe2]Cd0H-H20, obtained as an intractable material from the reactions of both (Et2NCH2CH2NEt2)CdCl2 and (Ph3P)2CdCl2 with K[TpMe2] in acetone, has been reported (89). However, the complex is not well characterized. 

Related to the zinc hydroxide complexes [TpRR]ZnOH, the hydrosul-fido complex [TpBut]ZnSH has been synthesized by the reaction of [TpBut]ZnH with H2S (80). However, the corresponding hydrosulfido complex [TpPh]ZnSH was not isolated from the reaction of the less sterically demanding derivative [TpPh]ZnBul with H2S, which gave preferentially the sandwich complex [TpPh]2Zn because of ligand redistribution (81b). 

The value of the tris(pyrazolyl)hydroborato complexes [TpRR ]ZnOH is that they are rare examples of monomeric four-coordinate zinc complexes with a terminal hydroxide funtionality. Indeed, [TpBut]ZnOH is the first structurally characterized monomeric terminal hydroxide complex of zinc (149). As such, the monomeric zinc hydroxide complexes [TpRR ]ZnOH may be expected to play valuable roles as both structural and functional models for the active site of carbonic anhydrase. Although a limitation of the [TpRR ]ZnOH system resides with their poor solubility in water, studies on these complexes in organic solvents... 

P(0)(0Me)H and [TpAr Me]Zn0P(0)(0R)2, respectively (Scheme 26) (35). In addition, [TpAr Me]ZnOH also cleaves activated esters and amides in a stoichiometric fashion, as illustrated in Scheme 27. Kitajima has described a similar amide cleavage reaction between the copper and nickel hydroxide complexes [Tp JMtOH) (M = Ni, Cu) and p-nitroacetanilide to give [TpPr 2]M p2-MeC(0)NC6H4N02 [Eq. (34)] (198). 

The reactions of carbon dioxide with zinc hydroxide complexes of the 3-t-butyl 5-methyl and 3,5-di-wo-propyl pyrazolyl derivatives were investigated. IR spectroscopy demonstrated formation of a bicarbonate derivative of the former complex and NMR showed a rapid reversible... 

The zinc acetate complex of tris(3-/-butyl-5-methylpyrazol-l-yl)borate was prepared as a structural model for carbonic anhydrase and comparison with the enzyme active site structures confirmed that the complexes are excellent structural models.239 A mononuclear zinc hydroxide complex can also be formed with the tris(pyrazolyl) borate ligand system as a structural model for carbonic anhydrase.240... 

Formation of aldehyde complexes from the zinc hydroxide complexes of tris(3-cumenyl-5-methylpyrazolyl)borate] was observed. The aldehyde ligands, such as pentafluorobenzaldehyde,... 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Based on Table 3.6, Cu and Pb may form Cu(OH)+ and Pb(OH)+ at pH 8.0 and 8.4, respectively, while Zn may form Zn(OH)+ at pH 9.0. However, Cd can form Cd(OH)+ at pH 10.1. This indicates that at normal pH ranges of arid and semi-arid soils, Cu and Pb, and to some extent, Zn may be present in their hydroxide complex ions, while Cd is less likely to be in the hydroxide form unless at higher soil pH (Table 3.5). Sauve et al. (1997) reported that free Cu activity in soil solution of Quebec and New York decreased with higher pH. Fotovat et al. (1997) reported that Zn(OH)2° and ZnCC>30 contributed to a considerable proportion of the total solution Zn (about 16-21%) in soils with high pH (7.56-8.99) from South Australia. Ma and Lindsay (1990) reported that the Zn activities measured in 10 Colorado arid soils can be expressed by the relationship with log K° = 5.7 0.38 (Fig. 3.1) ... 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst in toluene is exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex in toluene (the molar ratio of the initial butadiene to BuLi concentration was 500). The upper curve demonstrates that the percent trans content increased rapidly from 627. to 807. trans-1,4 as the temperature decreased from 75°C to 22°C. From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease in cis-1,4 content, the amount of vinyl unsaturation remaining at 5-87.. For the polybutadienes prepared using BuLi alone, there is only a very slight increase in the trans-1,4 content as the polymerization temperature is decreased. 

The only difference between the active oxidizing and a ferric porphyrin hydroxide complex is two electrons (scheme 4). Indeed, the electrochemical oxidation of hydroxy ferric tetra-mesitylporphyrin shows two reversible one-electron oxidations (40), and, in principle, use of water and an electrode should allow development of a system capable of catalytically oxidizing hydrocarbons. 

Copper may exist in particulate, colloidal, and dissolved forms in seawater. In the absence of organic ligands, or particulate and colloidal species, carbonate and hydroxide complexes account for more than 98% of the inorganic copper in seawater [285,286]. The Cu2+ concentration can be calculated if pH, ionic strength, and the necessary stability constants are known [215,265-267]. In most natural systems, the presence of organic materials and sorptive surfaces... 

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