REACTIVITY OF THE TRI-MU-HYDROXO-BIS[TRIAMMINECOBALT(III)] ION IN HALIDE MEDIA

The kinetics of acid hydrolysis of trans-[(H2O)(NH3)3Co(mu-OH)2Co(NH3) 3(H2O)]4+ in acidic chloride and bromide solutions was studied using s topped-flow and conventional spectrophotometric techniques, at an ioni c strength of 1.0 mol dm-3 and over the ranges 0.10 less-than-or-equal -to [H+] less-than-or-equal-to 0.50 mol dm-3, 0.10 less-than-or-equal- to [X-] less-than-or-equal-to 0.50 mol dm-3, 25 less-than-or-equal-to T less-than-or-equal-to 35-degrees-C and in binary aqueous cosolvent m ixtures containing up to 40% methanol by volume. The rate equation obs erved was of the form k(obs) = (k(c)[H+] + k(c)') [X-]. At 25-degrees- C the rate constants k(c) were 1.10 x 10(-2) and 1.22 x 10(3) dm6 mol- 2 S-1 in chloride and bromide solutions respectively for the [H+]-depe ndent term. The corresponding values for the [H+]-independent pathway, k(c)', were 4.5 x 10(-4) and 0.7 x 10(-4) dm3 mol-1 s-1. The activati on parameters in chloride solution were DELTAH(double dagger)(k(c)) = 51 +/- 14 kJ mol-1, DELTAS(double dagger)(k(c)) = -110 +/- 48 J K-1 mo l-1; DELTAH(double dagger)(k(c)') = 132 +/- 32 kJ mol-1 and DELTAS(dou ble dagger)(k(c)') = 136 +/- 105 J K-1 mol-1. At constant [H+] (0.5 mo l dm-3) the reaction was first order with respect to [Cl-] and [Br-] a nd the rate constants, k(d), were 4.5 x 10(-3) and 3.3 x 10(-3) dm3 mo l-1 s-1 respectively.