ChemInform Abstract: THE KINETICS OF AQUATION AND RACEMIZATION OF SOME CIS‐ CHLORO‐, HYDROXO‐, AND AQUABIS(ETHYLENEDIAMINE)(AMINE)COBALT(III) COMPLEXES

Abstract

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 .O M HNOz at 50-80°C. Kinetic parameters are (in the above order of A) 1O7k29s (sec-I), 7.4, 5.7, 1.3, 9.7; E, (kJ/mole), 103, 101, 130, 112; log PZ (sec-l), 11.89, 11.53, 16.04, 13.58 8, 38.0, 7.7, 1.7; E, = 150, 135, 152, 157; log PZ = 17.49, 16.18, 18.56, 18.87; ASzgsr = +82, +57, +102, +l08. Rates of racemization of the active hydroxo complexes cis-Co(en)z(A)(OH)2+ (A = NH3, C H~N H Z , 3,5-lutidine, imidazole) were measured similarly at pH = 8 ( p = 2.0, NaC104). The racemization of the hydroxo is ca. (3-4) X 103 faster than for the corresponding aqua complex. Kinetic parameters are 106km = 2. 55, 7.2, 30.1, 7.0; E, = 138, 123, 122, 128; log PZ = 18.58, 16.39, 16.85, 17.18; ASZss* = +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 ( p = 2.0, NaC104) at 50.6"C, and pK, data calculated from the pH versus k plot are 5.50, 5.65, and 6.40, respectively, for A.