A comparison of kinetic data for the reactions of superoxo-bridged dicobalt complexes with iodide in aqueous perchloric acid solutions

Abstract

The kinetics of the one-equivalent reactions of the superoxo-complexes (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺, (NH₃)₄Co·µ(NH₂,O₂)·Co(NH₃)₄⁴⁺, and (en)₂Co·µ(NH₂,O₂)·Co(en)₂⁴⁺ with iodide have been studied in aqueous perchloric acid solutions made up to µ= 2·0M with LiClO₄. A large excess of iodide was used so that first-order kinetics were observed. At constant hydrogen-ion concentrations, pseudo-first-order rate constants k_obs give the following dependence on [I^–]: k_obs=a[I^–]²+b[I^–] where at [H⁺]= 0·01M and 1·4°, a= 1·21, 12·5, and 350 I. mole^–1 sec.^–1 and b= 0·0024, 0·11, and 0·60 I. mole^–2 sec.^–1 for the complexes (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺, (NH₃)₄Co·µ(NH₂,O₂)·Co(NH₃)₄⁴⁺, and (en)₂Co·µ(NH₂,O₂)·Co(en)₂⁴⁺ respectively. Ion-pair formation (which is rapid) is believed to precede electron transfer in both cases. By varying the hydrogen-ion concentration over the range 0·01 to 2·0M it has been shown that each of the terms in a and b can be split into [H⁺]-independent and direct [H⁺]-dependent terms.

In preliminary runs on the reaction of iodide with the tri-bridged superoxo-complex (NH₃)₃Co·µ(NH₂,OH,O₂)·Co(NH₃)₃³⁺ at 25° and [H⁺]= 0·01M, overall rates are slower (by a factor of about fifty) than for the reaction of the mono-bridged complex. The reaction is much more rapid at higher hydrogen-ion concentrations however. The negative charge associated with the three bridging ligands is probably an important factor in explaining these rates.