Kinetics and Complex Formation of Molybdate with Catechol

Abstract

A spectrophotometric determination of binding in molybdate-catechol (H2L) solutions indicates the presence of two complexes. At 25° (±1), pH 7.3, and ionic strength 0.1 M, the stability constants for these two complexes areK1 = [MoO2(OH)2L2-]/[MoO42-][H2L] = 37.1 (±5) M-1 and K2[MoO2L22-]/[MoO2(OH)2L2-]· [H2L] = 1.52 (±0.4) × 103 M-1 Stopped-flow techniques were used to obtain the kinetic curves for the formation of these complexes; close-to-equilibrium relaxation theory was used for evaluation when consecutive reactions occurred. The apparent rate constants of formation and dissociation for each complex, under the same conditions given above are as follows: for the 1:1 (metal:ligand) complex, kiapp = (2.97 ± 0.25) × 102 M-1 sec-1, k-1app = 8.05 ± 0.30 sec-1; for the 1:2 complex, k2app = (5.4 ± 0.1) ×10-2 M1sec-1, k-2app = 0.36 ± 0.10 sec-1 The reduced stability of the mono relative to the bis complex is due chiefly to the larger dissociation rate constant of the former. This effect is probably due to the presence of two inner-sphere hydroxide ions in the 1:1 complex, the Π bonding of which to the molybdate center labilizes the bound catechol molecule. © 1973, American Chemical Society. All rights reserved.