Kinetics of base hydrolysis of cis-aminebromobis(ethylenediamine)-cobalt(III) Complexes in aqueous solution at 25 °C

Abstract

Rate constants, k_OH, are reported for the base hydrolysis of 11 penta-aminebromocobalt(III) complexes, cis-[CoBr(en)₂(NH₂R)]²⁺+[OH]^–→[Co(OH)(en)₂(NH₂R)]²⁺+ Br^–, at 25 °C and I= 0.1 mol dm^–3. For the straight-chain amines (R = Me, Et, Prⁿ, Buⁿ, n-pentyl, or n-hexyl) the values of K_OH are essentially constant at 82 ± 3 dm³ mol^–1 s^–1. These results are in contrast to previous reports that the Prⁿ derivative undergoes base hydrolysis 5–6 times faster than the methyl and ethyl derivatives. For the straight-chain amines the value of K_OH^Br/K_OH^Cl is ca. 62. Activation parameters for the base hydrolysis of the Prⁿ derivative are ΔH^‡= 89 ± 2 kJ mol^–1 and ΔS^‡= 96 ± 4 J K^–1 mol^–1 at 298 K. The positive ΔS^‡ is consistent with an S_N¹(CB) mechanism. Bulky amines such as cyclopropylamine and 2,2-dimethoxyethylamine lead to significant increases in the rates of base hydrolysis, probably due to steric acceleration of the ionisation step. With α, ω-aminoalkanols there is evidence for specific hydrogen-bonding effects leading to increased base-hydrolysis rates.