Comparison of the rates of substitution in [Ru(NH 3) 5H 2O] 3+ and [Os(NH 3) 5H 2O] 3+ by hexacyano complexes: substitution coupled to electron transfer

Abstract

A comparison of the rates of substitution (in water solutions, 20 °C) in [Ru(NH 3) 5H20] 3+ and [Os(NH 3) 5H 2O] 3+ by Fe(CN) 6 3−, Fe(CN) 6 4−, Ru(CN) 6 4− and Co(CN) 6 3− shows that the reaction of [Os(NH 3) 5H 2O] 3+ with Fe(CN) 6 3− is unique. Substitution in this and in the other cases takes place, as has been shown by others in earlier work, by rapid formation of ion pairs, followed by collapse of the ion pairs to inner-sphere products. For most of the systems the slight differences in the first-order rates governing inner-sphere formation can be accounted for by the tetranegative ions being more nucleophilic than the trinegative, and rates for Os(III) being somewhat slower than for Ru(III). Substitution in the special case takes place 1.5×10 3 times more rapidly than it does with Co(CN) 6 3− as entering group. The singularity of the Os(III)·Fe(III) system is ascribed to substitution coupled to 1 e − oxidation of Os(III) by Fe(III). A similar effect with [Ru(NH 3) 5H 2O] 3+ is minor because of the greater difficulty of 1 e − oxidation (a distinction which is borne out by comparison of the energies of the charge transfer spectra). In all cases but those involving Co(CN) 6 3−, strong absorption in the visible of charge transfer character is observed. In the systems with Fe(CN) 6 3− as nucleophile, interesting complications appear. On reacting with [Ru(NH 3) 5H 2O] 3+, Fe(CN) 6 3− produces two products by parallel paths. We conclude that the product showing absorption at higher energy, S h, is [Ru III(NH 3) 5·Fe III(CN) 6] while that at lower energy, S 1 is [Ru IV·Fe II(CN) 6]. The former is unstable with respect to the latter, and transforms to it in a process much slower than substitution. On reacting with [Os(NH 3) 5H 2O] 3+, Fe(CN) 6 3− leads initially to two products by parallel paths: S 1 which we conclude is [Os III(NH 3) 5·Fe III(CN) 6] and the major product, S i (i for ‘intermediate’), which is [Os IV·Fe II(CN) 6]. In this system S 1, [Os(III)·Fe(III)], is unstable to S i, [Os(IV)·Fe(II)] which eventually dispropionates to S h and [Os(III)·Fe(II)]. In following the reactions spectrophotometrically induction periods are observed in all of the reactions involving substitution (and/of redox change) with Os(III), except for the formation of S i, the appearance of which is first order without complications. The induction periods in all cases are greatly reduced when the solutions are acidified. Out working hypothesis, to be tested in further work, is that the induction periods are caused by linkage isomerization of the bridging CN −.