Kinetics of base hydrolysis of cis-aminebromobis(ethylenediamine)-cobalt(III) Complexes in aqueous solution at 25 °C
Abstract
Rate constants, kOH, 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 KOH 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 KOHBr/KOHCl 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 SN1(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.