Structural and mechanistic studies of co-ordination compounds. Part 33. Inner-sphere vs. outer-sphere mechanisms in the reductions of some trans-dianiono(tetramine)ruthenium(III) cations by chromium(II) and vanadium(II)

Abstract

The kinetics of the reduction of trans-[RuL¹AX]⁺[L¹= bis(ethane-1,2-diamine); AX = Cl₂, Br₂, I₂, or BrCl] and trans-[RuLCl₂]⁺[L = L²(3,7-diazanonane-1,9-diamine), L³(1,4,8,11-tetra-azacyclotetradecane), L⁴(1,4,8,12- tetra-azacyclopentadecane), L⁵ or L⁶(C-meso or C-rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane respectively)] by chromium(II) and vanadium(II) have been studied at 25.0 °C in aqueous acidic solutions of ionic strength 0.50 mol dm^–3. The chromium(II) reductions have been shown to proceed by an inner-sphere mechanism via the bridging ligand X^–. These reactions are very sensitive to steric effects with trans-[RuL⁵Cl₂]+ and trans-[ RuL⁶Cl₂]⁺ more reactive than the corresponding steric-free L³ complex by a factor of 76.5 and 58.0 respectively. For trans-[RuL¹AX]+ complexes, the second-order rate constants drop systematically in the following order of AX: I₂ > Br₂ ClBr BrCl > Cl₂(148, 38, 37, 31, and 15 dm³mol^–1 s^–1 respectively, after a statistical factor of two has been applied to the dihalogeno-complexes). For vanadium(II) reductions, the second-order rate constants, ranging from 1.21 × 10³ dm³ mol^–1 s^–1 for trans-[RuL¹Cl₂]⁺ to 1.27 × 10⁴ dm³ mol^–1 s^–1 for trans-[RuL¹I₂]⁺, are much greater than the ligation rates of vanadium(II). This, together with the observation that steric factors have relatively little effect on the reactivity, strongly supports an outer-sphere mechanism for these vanadium(II) reductions.