Abstract
A study by optical and pH methods indicates that the species present in solutions of vanadyl (V02 + ) ion and tartaric acid (H2T) below pH 2 can be formulated as 1:1 and 1:2 complexes of the types VOT and VOT22". The formation constants so determined are K¡ = ( )( +)/(V02+)(HT") = 12.5 and K% = (VOT22")(H+)/(VOT)(HT~) = 2.0 at 25° and ionic strength 0.25 Af. Temperature-jump and pressure-jump relaxation methods were used to study the reactions of tartaric acid and bitartrate anion with vanadyl ion. An initial-rate stopped-flow study was used to corroborate the rate constant for the formation of a 1:1 complex. Relaxation methods determine the forward rate constant for the reaction V02+ + HT“ ± VOT + H+ to be 170 M~1 sec-1 ± 20%. The rate constant determined by initial-rate stopped-flow methods is 185 Af"1 sec"1, the two values agreeing within experimental error. The forward rate constant for the reaction VOT + HT" s± VOT22" + H+ is 277 Af"1 sec"1 ± 10%. The rate constants for fully protonated tartaric acid are much smaller and were unable to be determined. The value determined for the rate of formation of the 1:1 complex, which is low relative
to that expected for normal substitution (i.e., that value equal to the product of the ion-pairing constant and the water- exchange rate on vanadyl), indicates that the rate-determining step m the reaction is the chelation process itself. Rather than being due to steric restrictions,this shift in rate-determining step is viewed as arising from protonation of the ligand binding site.