A kinetic and thermodynamic study of the unexpected comproportionation reaction between cis-[OsVIIIO4(OH)2]2− and trans-[OsVIO2(OH)4]2− to form a postulated [OsVIIO3(OH)3]2− complex anion

Abstract

A kinetic study of [OsO₄] reduction by aliphatic alcohols (MeOH and EtOH) was performed in a 2.0 M NaOH matrix at 298.1 K. The rate model that best fitted the UV-VIS data supports a one-step, two electron reduction of Os^VIII (present as both the [Os^VIIIO₄(OH)]⁻ and cis-[Os^VIIIO₄(OH)₂]²⁻ species in a ratio of 0.34 : 0.66) to form the trans-[Os^VIO₂(OH)₄]²⁻ species. The formed trans-[Os^VIO₂(OH)₄]²⁻ species subsequently reacts relatively rapidly with the cis-[Os^VIIIO₄(OH)₂]²⁻ complex anion to form a postulated [Os^VIIO₃(OH)₃]²⁻ species according to: cis-[Os^VIIIO₄(OH)₂]²⁻ + trans-[Os^VIO₂(OH)₄]²⁻2[Os^VIIO₃(OH)₃]²⁻. The calculated forward, k₊₂, and reverse, k₋₂, reaction rate constants of this comproportionation reaction are 620.9 ± 14.6 M⁻¹s⁻¹ and 65.7 ± 1.2 M⁻¹s⁻¹ respectively. Interestingly, it was found that the postulated [Os^VIIO₃(OH)₃]²⁻ complex anion does not oxidize MeOH or EtOH. Furthermore, the reduction of Os^VIII with MeOH or EtOH is first order with respect to the aliphatic alcohol concentration. In order to corroborate the formation of the [Os^VIIO₃(OH)₃]²⁻ species predicted with the rate model simulations, several Os^VIII/Os^VI mole fraction and mole ratio titrations were conducted in a 2.0 M NaOH matrix at 298.1 K under equilibrium conditions. These titrations confirmed that the cis-[Os^VIIIO₄(OH)₂]²⁻ and trans-[Os^VIO₂(OH)₄]²⁻ species react in a 1 : 1 ratio with a calculated equilibrium constant, K_COM, of 9.3 ± 0.4. The ratio of rate constants k₊₂ and k₋₂ agrees quantitatively with K_COM, satisfying the principle of detailed balance. In addition, for the first time, the molar extinction coefficient spectrum of the postulated [Os^VIIO₃(OH)₃]²⁻ complex anion is reported.