The solution structure and reactivity of decavanadate

The oxygen-exchange reaction of V₁₀O with bulk water has been followed by time-dependent ¹⁷O-NMR spectroscopy (buffered solutions, pH ～ 5.5, [V₁₀]_total ～ 0.17m, T = 298 K). It is shown that all seven structurally different sites of O-atoms are kinetically similar but, in contrast to earlier studies, not identical (6 h ? ‘t_1/2’ ? 11 h). The kinetic similarity of the various structural sites implies the some (but not full) O scrambling is involved. Two possible mechanisms with a ‘half-bonded’ and an ‘open’ intermediate are discussed in detail to interpret the experimental results. A computer simulation of the exchange reaction based on these models is presented. It is shown that the ‘half-bonded-intermediate’ mechanism is consistent with the experimental data and the following parameters are calculated: formation of the intermediate: k₁ = 5.8 · 10^?3 s^?1, k_?1 = 6.7 · 10^?2 s^?1, [intermediate]_∞ ≈ 8%; all activated O-atoms exchange within the lifetime of the intermediate (τ ～ 15 s), and the calculated exchange rate of the intermediate (k₂ ? 0.60 s^?1) is consistent with earlier assumptions (k₂ ≈ 0.5 s^?1). It is shown that a simulation based on the ‘open-intermediate’ mechanism results in kinetic parameters which are not consistent with the kinetics of the formation of cyclic metavanadates ((VO)_n, n = 4,5) from decavanadate, since the required formation rate is by a factor ～ 10² too fast, and the equilibrium concentration of metavanadates is by a factor of ～ 2 too large (under the conditions of the O-exchange experiments of decavanadate (T = 298 K, [V₁₀]_total ≈ 0.17m, pH ～ 5.55) the total amount of metavanadates present is ～ 8%, with [(VO)₄]/[(VO)₅] ～ 4:1; a qualitative analysis of the kinetics of the formation of metavanadates (v_o kinetics; the exact mechanism of the back-reaction (at least second-order) is not known with certainty) leads to k₁ ? 4·10^?5 s^?1). O exchange of decavanadates via equilibrated metavanadates would lead to full scrambling of the O sites and is not consistent with the observed differences in the exchange rates. From the qualitative kinetic parameters of the metavanadate formation kinetics, it can be concluded that any contribution of an ‘open’ or an ‘metavanadate’ mechanism is of the order of 1–2% at most.