Acid catalyzed aquation

Metal ions can assist in the dissociation (hydrolysis) of complexes containing multidentate ligands. The metal ion may not necessarily complex with the detached ligand, for example, in the metal-assisted acid-catalyzed aquation of Cr(C204)3. Ref. 112. Usually however the metal ion removes and complexes the ligand as in... 

The acid-catalyzed aquation of iron(III)-(substituted)oxinate complexes involves iron oxygen bond breaking and concomitant proton transfer in transition state formation. The latter aspect contrasts with the much slower acid-catalyzed aquation of hydroxamates, where proton transfer seems not to take place in the transition state. Reactivities, with and without proton assistance, for various stages in dissociation of a selection of bidentate and hexadentate hydroxamates, oxinates, and salicylates are compared and discussed—the overall theme is of dissociative activation. ... 

The data of Table IV strongly suggest that water and SCN- are reacting in a competitive fashion, with a reactive intermediate generated in or after the ratedetermining step of the acid-catalyzed aquation. Equations 19, 20, and 21 represent a plausible mechanism for generation of the reactive intermediate, assumed to be Co(CN)5-2, and its competitive reactions with water and SCN . 

The kinetic parameters obtained in the absence of SCN " are of some inherent interest. The acid-catalyzed aquation of ligands which are conjugate bases of weak acids has been observed in a number of other systems (/), but previous studies... 

Several alternative interpretations may be suggested to explain the above results. First, it is possible that the scavenger ions are reacting, not with Co(CN)5 2, but with some other reactive intermediate such as the species Co(CN)5N40r 2 mentioned above. Secondly, there is the possibility that Co(CN)5-2 is generated in both the acid-catalyzed aquation and the HNO2 reaction, but that the two reactions may produce different isomeric forms of Co(CN)5-2, one form... 

The acid catalyzed aquation of the chelated carbonato complexes involves three steps (i) the initial rapid protonation pre-equilibrium of the exocyclic oxygen, (ii) carbonate ring opening, and (iii) decarboxylation of the resulting bicarbonate complex [see Eq. (30)]. 

The acid-catalyzed aquation of cis-[Cr(mal)2(OH2)2] to give [Cr(mal)(OH2)4] has been studied in detail. The Cu -, Ni"-, Co - and Zn -catalyzed dissociation of the cis-diaquabis(malonato)chromate(III) ion has also been recently investigated using perchloric acid solutions. Under these conditions kobs = where k<,bs is the observed first order... 

All the cyanoaquochromium(III) complexes studied exhibit an acid-catalyzed aquation pathway which involves first the protonation of a cyanide ligand, described by the equilibrium constant Ki, and then the aquation of the singly-protonated species, described by the rate constant fci. In Fig. 7, the logarithm of the product kiKi, which is proportional to the observed reaction rates and which is accurately known for each reaction, is plotted as a function of the number of ligand cyanide groups in the complex. Also, included in the figure are plots of log Ki and log ki vs. the number of coordinated cyanides. 

Acid-catalyzed aquation has already been covered at appropriate points in Section 5.7.1. Most of the present section deals with catalysis of removal of halide from cobalt(III) with the aid of mercury(II). The final paragraphs of the section deal with the question of catalytic removal of a variety of groups and the relation of the results to the question of the possible intermediacy of transient five-coordinated intermediates. 

Such reactions have already cropped up in Sections 5.7.1. and 5.7.3, for both acid-catalyzed aquation and mercury(II)-assisted aquation are reactions in which the coordinated ligand plays a central role. Other manifestations of ligand reactivity are dealt with in this section. 

Following the synthesis of the new edta complex, Na3[Cr(L)-(CN)20H2] 4H20 (H4L = edta) in which edta is terdentate with three car-boxylate groups uncoordinated, the kinetics of the acid-catalyzed aquation of the first cyanide group has been measured. The loss of the second cyanide group is rapid, and acid catalysis involves protonation of one of the uncoordinated carboxylate groups ... 

Table 6.6. Rate Data at 298.2 K for the Acid-Catalyzed Aquation of Substituted tris(Malonato)Chromium(III)...

Acid-catalyzed aquation of [Co(NH3)sOC02], cis-j8-[Co(edda)C03] , and [Co(nta)C03] has been reinvestigated using rapid scan spectrophotometry.Direct spectral evidence for the participation of proton-ated and ring-opened carbonato species was obtained. The spectral observations are consistent with previously suggested mechanisms for the decarboxylation of monodentate and bidentate carbonato complexes. 

Table 2.6.3 contains the rate constants of some uncatalyzed and acid catalyzed aquations. 

Figure 2.6.3 depicts the usual pattern of an acid catalyzed aquation. 

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