Oxolation mechanism

Oxolation occurs in aqueous solutions when there is no coordinated aquo group i.e., either in higher valent metals or in solutions at higher pH. Two basic oxolation mechanisms exist. First, if the metal coordination is not fully satisfied then rapid nucleophilic addition can occur in order to achieve saturation (Equation (11a)). 

Decavanadate species, written generally as Vio02s (OH). are the predominant V(V) species at moderate acidities [lOJ and may also form by polymerization of [VOjfOH ) precursors. (See, e.g.. Fig. 5.) Structures of several common polyanions are shown in Figs 6a and 6b. More compact polyanions (Fig. 6a) are formed when the kinetics of condensation via A,v or oxolation mechanisms are rapid, whereas slower kinetics result in more open structures (Fig. 6b) and generally allow the formation of clear gels when an acid is added. 

The condensation reactions occur only through the mediation of sylanol groups generated by hydrolysis. There are two main opportunities. The condensation can be carried out by oxolation or olation mechanisms. The oxolation occurs with the participation of two sylonol groups. In the course of the reaction a hydroxo ligand is exchanged for an oxo one [8] ... 

Finally, PdO particles are spontaneously formed from palladium hydroxide Pd(OH)2 via dehydration hy internal oxolation in agreement with the positive value of the partial charge of the water molecule (5h20 = 0.03). Figure 13.3 summarizes the overall mechanism leading to the formation of PdO colloidal particles. 

Figure 13.19 summarizes the reaction mechanism starting from Sn(OH) + or Sn(OH)e in acidic media in alkaline media. As in the case of Pd, Sn02 oxide is spontaneously formed by dehydration due to an internal oxolation reaction promoted by a strong polarization of the O-H bond of the hydroxide. Thermodynamically stable species with respect to pH are presented in Fig. 13.20. Various molecular cationic species with different hydroxylated levels are possible in an acidic medium, whereas only Sn(OH)g is expected for a basic pH.

Fe(N03)3 solutions. Precipitates formed from 6.25 x I0 and 6.25 x lO M Fe(N03)3 solutions at 24 and 55 C showed the characteristic pattern of a-FeOOH, whereas in 6.25 x lO M, Fe(N03)3 solution at 24°C an amorphous precipitate was formed. a-Fe203 precipitated at 90 °C. The same authors proposed (Fig. 23.2) the mechanism of hydrolysis-precipitation from Fe(N03)3 solutions, consisting of the hydrolysis to monomers (A) and dimers (B), reversible, rapid growth of small polymers (C), the formation of slowly reacting large polymers (D), and polymerization and oxolation with the formation of a solid phase (E). XRD, FT-IR, and Mbssbauer spectroscopies were used to characterize hydrolytical solid products obtained by the hydrolysis of O.l M Fe(N03)3 aqueous solutions at 90°C in the presence of hexamethylenetetramine (HMTA) [24,25]. HMTA generates OH ions at an elevated temperature in accordance with the chemical reactions... 

For coordinatively saturated metals in the absence of catalyst, hydrolysis and condensation both occur by nucleophilic substitution (S ) mechanisms involving nucleophilic addition (A,y) followed by proton transfer from the attacking molecule to an alkoxide or hydroxo-ligand within the transition state and removal of the protonated species as either alcohol (alcoxolation) or water (oxolation) [8,13,113] ... 

Hydrolysis and polycondensation reactions usually occur simultaneously and the reaction rates depend on the type of precursor as well as on reaction conditions such as pH, temperature, and ionic strength [8]. Hydrolysis of alkoxy precursors (=M-OR , where M is the metal atom and R is the alkyl group) occurs by water due to electrophilic reaction in the presence of acid catalyst or by nucleophilic substitution of alkoxy groups in the presence of base catalyst [3,9]. The hydrolyzed precursors (=M-OH) can react either with alloy precursors (alcoxolation) or with other hydrolyzed precursors (oxolation). In both cases the mechanism is nucleophilic substitution or nucleophilic addition for which the result is polycondensation [3,10]. The reactions that describe the sol-gel process are as follows ... 

Prolonged rellux of solutions around pH 2.5 leads, after about 20h, to sols of monoclinic ZrO . (distorted fluorite structure) [100,101]. It does not appear that the tetramer [Zr4(OH)i6(OH2)g]° is involved in the reaction. Indeed, the geometry of the coordination polyhedron of zirconium is not the same in the tetramer (antiprismatic, 8-coordination) and in monoclinic zirconium (7-coordination, Figure 3.22). In addition, acidification of the medium by thermohydrolysis (the pH decreases from 2.5 to 0.4 after 40 h reflux) [100] probably destroys the tetramer, and it is likely that lire growth of the solid takes place via a mechanism of acidic oxolation between monomers (see Chapter 4). 

This reaction mechanism is oversimplified because gelation of vanadic acid requires the presence of traces of V(1V), which appear spontaneously during acidification with an ion-exchange resin or with addition of alcohol [58,62]. The role of V(1V) as a catalyst is not quite clear yet [63]. It may act as an initiator of the initial condensation, either by olation of vanadic complexes or in the oxolation of the chains. Indeed, the nucleophilic power of the hydroxo ligands in the. species formed by V(IV), ]VO(OH)2(OH2)3l , is probably greater than that of the hydroxo... 

Olation takes place only if the aquo ligand exists as such in the coordination sphere of the cation (see Chapter 2). The reaction leads to formation of a solid phase of the hydroxide M(OH). The reaction is the only one taking place (meaning that the hydroxide is stable), as long as (HzO) is negative in the solid. If (HzO) is positive, oxolation by internal dehydration may become a competing mechanism. 

Assuming tliat there are no labile groups within the coordination sphere of the reactants, the oxolation reaction mechanism is associative. The coordination of the transition state must increase by one unit before a leaving group can be released. 

Experimentally, oxolation is often much slower than olation. For example, condensation via oxolation of silicic acid Si(0H)4 around pH 3 is much slower than condensation via olation of aquo-hydroxo complexes of titanium and zirconium. It is possible (hat (he difference in mechanism would make olation intrinsically faster than oxolation. A comparison is difficult becau.se it would require the... 

If Xm Xoi.z hydroxides are not stable. Oxyhydroxides or oxides are obtained via dehydration if olation and oxolation are competing mechanisms. If there is no... 

Although qualitative, the proposed mechanism stresses that the formation of vanadium oxide does not stem from an oxolation reaction alone. This is also true for other elements [Sn(lV), Sb(V), W(VI), etc.). The formation of solid phases of metal oxides by precipitation always involves at least one olation step, because oxolation alone leads only to the formation of polyanipns (see Chapter 2). 

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