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

Aquation rates for cis ‐CoCl(en) 2 (A) 2+ (A = 3,5‐lutidine, imidazole, N ‐methylimidazole, benzimidazole) have been determined by halide release titration in 1.0 M HNO 3 at 50–80°C. Kinetic parameters are (in the above order of A) 10 7 k 298 (sec −1 ), 7.4, 5.7, 1.3, 9.7; E a (kJ/mole), 103, 101, 130, 112; log PZ (sec −1 ), 11.89, 11.53, 16.04, 13.58; Δ S 298 ‡ (J/°K· mole),−26, −32, +54,+7. Rates of racemization for active cis ‐Co(en) 2 (A)‐(OH 2 ) 3+ were measured spectropolarimetrically in 0.1 M HClO 4 , 0.02 M Hg 2+ for A = ammonia, cyclohexylamine, 3,5‐lutidine, N ‐methylimidazole. Kinetic parameters are (units as above) 10 9 k 298 = 1.8, 38.0, 7.7, 1.7; E a = 150, 135, 152, 157; log PZ = 17.49, 16.18, 18.56, 18.87; Δ S 298 ‡ = +82, +57, +102,+108. Rates of racemization of the active hydroxo complexes cis ‐Co(en) 2 (A)(OH) 2+ (A = NH 3 , CH 3 NH 2 , 3,5‐lutidine, imidazole) were measured similarly at pH = 8 (π = 2.0, NaClO 4 ). The racemization of the hydroxo is ca. (3–4) × 10 3 faster than for the corresponding aqua complex. Kinetic parameters are 10 6 k 298 = 2.55, 7.2, 30.1, 7.0; E a = 138, 123, 122, 128; log PZ = 18.58, 16.39, 16.85, 17.18; Δ S 298 ‡ = +102,+60, +69,+76. Racemization rates for aquahydroxo mixtures (A = CH 3 NH 2 , 3,5‐lutidine, imidazole) were also determined in the pH range of 4–8 (θ = 2.0, NaClO 4 ) at 50.6°C, and p K a data calculated from the pH versus k plot are 5.50, 5.65, and 6.40, respectively, for A.