Reactions of the pentacyanocobaltate(II) ion: II. Liquid phase hydrogenation and decomposition

Abstract

In connection with the homogeneous catalytic activation of molecular hydrogen by pentacyanocobaltate(II) ions in aqueous solution, several features of the complex have been investigated. The reaction between hydrogen and pentacyanocobaltate(II) results in an equilibrium, the position of which is independent of the total Co concentration. From the changes of the equilibrium constant with temperature a reaction enthalpy of 11 kcal/mole H 2 has been calculated. The results of kinetic investigations of the hydrogen absorption show that, similarly to the decomposition of [Co(CN) 5] 3- under the influence of water, the rate-determining step is of second order with respect to cobalt. If the hydrogenation is assumed to be a termolecular reaction it shows zero energy of activation. It is, however, also possible to explain the kinetic results by means of other reaction schemes. The most satisfactory is that in which in a rapidly established equilibrium, pentacyanocobaltate(II) first forms a small amount of dimer which subsequently either absorbs hydrogen to form a hydrogenated monomer complex, or decomposes under the influence of water.