The kinetics of aquation and racemization of some cis‐ chloro‐, hydroxo‐, and aquabis(ethylenediamine)(amine)cobalt(III) complexes

Abstract

Aquation rates forcis‐CoCl(en)2(A)2+ (A = 3,5‐lutidine, imidazole, N‐methylimidazole, benzimidazole) have been determined by halide release titration in 1.0 M HNO3 at 50–80°C. Kinetic parameters are (in the above order of A) 107k298 (sec−1), 7.4, 5.7, 1.3, 9.7; Ea (kJ/mole), 103, 101, 130, 112; log PZ (sec−1), 11.89, 11.53, 16.04, 13.58; ΔS298‡ (J/°K· mole),−26, −32, +54,+7. Rates of racemization for active cis‐Co(en)2(A)‐(OH2)3+ were measured spectropolarimetrically in 0.1 M HClO4, 0.02 M Hg2+ for A = ammonia, cyclohexylamine, 3,5‐lutidine, N‐methylimidazole. Kinetic parameters are (units as above) 109k298 = 1.8, 38.0, 7.7, 1.7; Ea = 150, 135, 152, 157; log PZ = 17.49, 16.18, 18.56, 18.87; ΔS298‡ = +82, +57, +102,+108. Rates of racemization of the active hydroxo complexes cis‐Co(en)2(A)(OH)2+ (A = NH3, CH3NH2, 3,5‐lutidine, imidazole) were measured similarly at pH = 8 (π = 2.0, NaClO4). The racemization of the hydroxo is ca. (3–4) × 103 faster than for the corresponding aqua complex. Kinetic parameters are 106k298 = 2.55, 7.2, 30.1, 7.0; Ea = 138, 123, 122, 128; log PZ = 18.58, 16.39, 16.85, 17.18; ΔS298‡ = +102,+60, +69,+76. Racemization rates for aquahydroxo mixtures (A = CH3NH2, 3,5‐lutidine, imidazole) were also determined in the pH range of 4–8 (θ = 2.0, NaClO4) at 50.6°C, and pKa data calculated from the pH versus k plot are 5.50, 5.65, and 6.40, respectively, for A.