Abstract
Rate constants for the complexation of molybdate and tungstate with catechol derivatives have been determined at 25 ± 1 °C and ionic strength 0.5 M (NH4CI) by the approach-to-equilibrium technique on a stopped-flow apparatus.
Ligands studied were 1,2,4- and 1,2,3-trihydroxybenzene (pyrogallol), 3,4,5-trihydroxybenzoic acid (gallic acid), 3,4-dihydroxyphe- nylalanine (L-Dopa), and [3,4-dihydroxyphenyl]-2-methylaminoethanol (D-epinephrine). The formation of the mono (1:1) complex is more rapid for protonated than for unprotonated oxyanion. From the hydrogen ion dependence of the relaxation time it was determined that reactions of completely deprotonated ligand with completely deprotonated oxyanion, and com- pletely protonated ligand with protonated oxyanion, do not contribute, within experimental error, to the observed rate of com- plexation. The relaxation times (standard deviations ±5%, except for pyrogallol and 1,2,4-trihydroxybenzene ±10%) consist of acid-independent and -dependent parts which contain kinetically indistinguishable terms for which upper limits could be deduced by setting all but one term equal to zero. Some of the upper limits exceed diffusion control allowing minimum limits to be set for the terms previously set equal to zero. For some pathways the upper and lower limits are approximately the same, leading to the actual value within experimental error and the uncertainties in the associated acid dissociation constants and estimated diffusion controlled rate constants. For molybdate and tungstate complexations with these and other ligands a trend in complex formation rate constant with basicity occurs. Namely, if the oxyanion is protonated the most basic ligand is most reactive. If the oxyanion is unprotonated, the least basic ligand is most reactive. The fastest rate of complex formation occurs when the protonated oxyanion reacts with the most basic ligand fully deprotonated at the binding sites. These trends are ex- plained by assuming that the tetrahedral unprotonated oxyanion reacts by an addition mechanism, and the (postulated) octa- hedral protonated oxyanion reacts by a substitution mechanism. Ligand basicity then controls complex formation in substitu- tion by assisting elimination of the OH" groups to be replaced through a hydrogen-bond-transfer mechanism, but hinders ad- dition through the same effect. For 1,2,4-trihydroxybenzene the kinetics of formation of mono and bis complexes has been de- termined at ionic strength 0.1 M. Unlike the formation of the mono complex, the formation rate of the bis complex decreases with decreasing pH. This effect is also explained by the fact that the reactive metal-containing species in the higher order com- plex formation step is already octahedrally coordinated. There is no conversion to octahedral form upon protonation, and the influence of ligand protonation dominates the process.