Metal-hydroxo complexes

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. 

Recent studies indicate that the adsorption of metal ions is controlled only in part by the concentration of the free (aquo) metal ion of considerable importance is the ability of hydroxo and other complex ions and molecules to adsorb. There have been two apparently divergent approaches to describe the role played by hydroxo metal complexes in adsorption at solid-aqueous electrolyte interfaces. Matijevic et al. (9) have proposed that specific hydrolysis products—e.g., Al8(OH)2o4+ in the A1(III)-H20 system, are responsible for extensive coagulation and charge reversal of hydrophobic colloids. It has also been demonstrated by Matijevic that the free (aquo) species of transition and other metal ions... 

Woolley230 has found that a number of hydroxo metal complexes of the macrocycle (64) are active in the hvdration of C02 and acetaldehyde. Electronic spectroscopy231 and X-ray crystallography232 suggest coordination numbers in aqueous solution of five for [CoL]2+ and [ZnL]z+, and six for [NiL]2+ and [CuL]2+. For the [ZnL]2+ system, the p/Ca of the coordinated water molecule is 8.69 at 25 °C and the hydroxo complex is a reasonable catalyst for C02 hydration. The complex [Cu(glycylglycinate)OH] is also active in C02 hydration.233... 

Brown, P.L. Sylva, R.N. Ellis, J. An equation for predicting the formation constants of hydroxo-metal complexes. J. Chem. Soc., Dalton Trans. 1985, (4), 723-730. 

In the case of Fe(III), hydrolysis can go beyond the uncharged species Fe(0H)3(H20)3(s) to form anions such as the ferrate(III) ion, probably [Fe(OH)4 2H2O]. All hydrated ions can in principle donate a larger number of protons than that corresponding to their charge and can form anionic hydroxo metal complexes but, because of the limited pH range of aqueous solutions, not all elements can exist as anionic hydroxo or 0x0 complexes. 

Figure 7.3. Solubility of oxides and hydroxides. Free metal-ion concentration in equilibrium with solid oxides or hydroxides. The occurrence of hydroxo metal complexes must be considered for evaluation of complete solubility.

Before discussing the preparation of late transition metal complexes resulting from the activation of O-H bonds by late transition metal complexes, we wbl describe metathesis methods for the preparation of hydrido(hydroxo), hydrido(alkoxo), and hydrido(carboxylato) complexes. Though many methods of preparation of transition metal hydroxides, alkoxides, etc. by a metathesis reaction have been reported [1], only a limited number of examples of the preparation of hydrido(hydroxo), hydri-do(alkoxo) complexes etc. by metathesis are available. 

Otsuka and coworkers—addition of ligands to Pt and Rh complexes to facilitate water activation. Most researchers in the water-gas shift field focused their research primarily on the activation of CO through coordination that facilitated the nucleophilic attack by OH- or H20. In addition to this, Ostuka and coworkers28,40,47,55,56 added a new approach. It was based on a strategy that induces two-electron transfer from a low valent metal compound to a H20 molecule that leads to a hydrido-hydroxo-metal species, M + H20 <-> MH(OH). In so doing, they predicted that nucleophilic attack by the OH- on the coordinated CO would be more facile relative to the neutral H20 molecule. 

RhHL3 was found to be an even more efficient catalyst than its Pt counterpart. However, pyridine tended to stabilize the hydrido-hydroxo-metal species relative to acetone, such that pyridine promoted the water-gas shift rate. Otsuka and coworkers28,40 tested a wide range of Rh complexes. Results of reaction testing for both Pt[P(iPr)3]3 and the Rh complexes are provided in Table 13. The conditions were 0.1 mmol catalyst, 2.0 ml H20, 20 kg/cm2 CO, and 5 ml solvent at 100 °C for 18 hours. 

The observed protonation constant describing the proton transfer on the hydroxo oxo complex, is a function of the total metal complex concentration, [MJPoJoh = d[MOH]/[MMt, therefore given by Eq. (17)... 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

The marked increase in the acidity of water when it becomes metal-coordinated, as shown in Table 6.5, has very important ramifications. Many metal complexes will be involved in an aqua-hydroxo equilibrium in the common pH region of 3 to 11. Since the hydroxo form often... 

A route for designing Gd(HI) complexes whose relaxivity depends on the presence of lactate, is provided by the ability shown by some hexa- or hepta-coordinate chelates to form ternary complexes with a wide array of anionic species (154-161). The interaction between the coordinatively unsatured metal complex and lactate involves the displacement of two water molecules coordinated to Gd(III) ion with the two donor atoms of the substrate, thus leading to a marked decrease in the relaxivity. Lactate is a good ligand for Gd(IH) ion because it can form a stable 5-membered ring by using the hydroxo and carboxylic oxygen donor atoms (Fig. 19). 

Akita and coworkers established a dehydrative condensation procedure, starting from hydroxo metal precursors containing the hydrotris(3,5-diisopropylpyrazolyl) borato ligand, and they were able to obtain dinuclear (/r-peroxo)Pd complexes, as indicated in Scheme 4. With the same procedure peroxo and hydroperoxo species (for Pd and Rh) and alkyl peroxides (for Mn, Co, Ni and Pd) complexes may be obtained. 

The photochemistry of Zn+—(CH4) and Zn —(CH3OH) complexes has been studied in detail In that context, the possibility of the formation of a metal-hydroxo insertion complex [HO—Zn—CH3]+ (the isomer of [Zn—(CH30H)]+) has also been discussed . In other series of studies, the mechanism of dimethylzinc zinc monoethyl cation, diethylzinc diethylzinc dimer and dipropylzinc photolysis has been investigated by photoionization techniques. It was the study of Borsella and Larciprete that first observed different gas phase photodissociation mechanisms for Mc2Zn and Et2Zn by using one- and two-color multiphoton ionization combined with TOE MS. 

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