Abstract
The kinetics of complexation of dioxovanadium(V) by catechol and a series of catechol derivatives have been stud- ied at 25.0 ± 0.5° and ionic strength 1.0 M (CIO4-) in acidic media (0.2-1.0 M HCIO4). Upon mixing acidic solutions of vanadium(V) and catechol (or catechol derivatives) a highly colored complex is formed which subsequently disappears due to oxidation of the ligand. Rate constants for complex formation (k f) and dissociation (kr) have been determined by stopped flow. The rate constants show no detectable [H+] dependence; it is therefore concluded that HOV02+ is not important, and the reactive species are V02+ and fully protonated ligand. Both k; and kr are sensitive to the nature of the complexing lig- and. For catechol, pyrogallol, 1,2,4-benzenetriol, L-dopa, and epinephrine, the respective krvalues are 1.84 X 104, 4.36 X 104, 9.91 X 104, 1.12 X 104, and 1.70 X 104 M~' sec-1 and for Arr 41.9, 8.39, 10.8, 55.2, and 66.6 sec-1. The values of kt represent lower limits on the substitution rate constants. For example, with catechol, the vanadium(V) water exchange rate constant, k0, is kp > 6 X 104 sec-1. This value is greater than ko for V(IV) and is explained by electrostatic effects on the bonding to the remaining water molecules; namely, the additional oxide ligand compensates for the higher charge on the cen- tral metal atom. The order in kf and K, the complex stability constant, is the same, and may also be explained by an electro- static effect. Substitution rate constants of V(V) in media of pH 0-9 lie within the narrow range 1 to 10 X 104 A/-1 sec-1 due probably to rate controlling loss of coordinated aquo ligands.