Proton ambiguity

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

The stoichiometiy of the eomplexes, which cannot be determined from the equilibrium data because of the proton ambiguity (Th(0H)(HjP04)2 = Th(HP04)(H2P04), has instead been estimated based on partial atomie eharges. Despite the eare taken both in the experiments and the data analysis, the present review does not aeeept the stoichiometiy and the equilibrinm constants proposed the reasons are there is a large nnmber of parameters in the model nsed and only one single model... 

In this set, the and paths are indistinguishable from the and 2 paths, respectively, and can be eliminated from the mechanistic scheme [the so-called proton ambiguity (2)]. The overall rate law consistent with this modified set, (29), (30), (31), (34), is given in Eq. (35). 

From studies of the high-spin monophenolate complexes of Fe using the temperature-jump method, it is suggested that pathways involving [Fe(H20)e] + and [Fe(H20)5(OH)] + ions i.e. proton ambiguity) can be distinguished by measuring values of for the phenolate dissociation reactions. The pertinent data are collected in Table 16. Comparison with related systems shows that is rather different for the two reactions [Fe(H20)8] ++L and [Fe(H20)5(0H)]2+ + HL. 

Despite its common occurrence and importance, FeCOHj) was the last of the air-stable first-row transition-metal ions to have its water exchange rate determined. The difficulty is the tendency of FeCOHj) to hydrolyze and polymerize in dilute aqueous acid, forming Fe(OH2)5(OH) and (H20)4Fe(0H)2Fe(0H2)4 , respectively, as the major species. There have been numerous kinetic studies of substitution on Fe(OH2)5 but flie results were difficult to interpret because of the lack of a water exchange rate until 1981 and because of the proton ambiguity discussed below. 

The proton ambiguity refers to the fact that the [H J dependence of k does not allow one to separate kJCJK from kj in the first case, or kJK. from k- K in the second case. This results because the transition states, Fe(OH) HL and FefOHj) L, both contain one ionizable proton that is in different sites. The kinetics can only give the composition of the transition state, not its structure. 

The specific assignment of A and k" can be made by comparison to the theoretical expressions for the appropriate case, except for the proton ambiguity problem. 

If reactants are involved in protolytic equilibria, there is often a proton ambiguity as to the state of protonation of the actual reacting species. In the case of the reaction of peroxynitrite ion with carbon dioxide, Lymar and Hurst were able to show that the reactants are COj and ONOj". The overall system can be described by the following scheme ... 

The mono complex of Ga(III) with 5-nitrosalicylate in water is formed in two pairs of proton-ambiguous paths. Attributing the observed rate exclusively to the two reactions involving the monoprotonated ligand on the one hand and Ga and Ga(OH) on the other leads to upper limits for the rate constants of 4.1 X 10 and 6.4 x 10 dm mol" s" respectively. Comparison with other results suggests an associative mechanism. 

Further evidence is also provided for an associative mechanism in the formation of complexes at vanadium(III). Rate constants are given for the reaction of with HL and HsL" (7.0 x 10 and 3.3 dm mol" s respectively), where the ligand is p-aminosalicylic acid (H2L), but the proton ambiguity precludes values for reaction with the nonpolar H2L and the zwitterionic H2L. ... 

Unfortunately, direct analysis of the reaction parameters of tetrahedral boronates with these acidic ligands is complicated by the presence of reaction pathways, which are kinetically indistinguishable due to proton ambiguity. Therefore, the bias of the literature towards examining the complexation of ligands with neutral boronic acids necessitates that we discuss this first in order to develop a rounded perspective of the mechanism. 

Scheme 16 As the reaction kinetics of tetrahedral borates with most ligands cannot be followed due to the problems associated with kinetically indistinguishable pathways and proton ambiguity, studies have examined the biruling of boric acid with bidentate ligands in a 1 2 complex. By considering the change from a 1 1 to a 1 2 complex a tetrahedral structure is enforced at boron. This allows parallels to be made with the complexation of other tetrahedral boronate anions. Yoshimura s proposed transition state is depicted here, illustrating the complexation of boric acid with chromo tropic acid. ...

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