Reactions of metal complexes in solution. Part III. The acid-catalysed aquation of trans-dinitrobis-(αα-bipyridyl)cobalt(III) nitrate and the trans–cis-isomerisation of the nitroaquobis-(αα-bipyridyl)cobalt(III) ion

Abstract

The acid-catalysed aquation of trans-dinitrobis-(αα-bipyridyl)cobalt(III) nitrate has been studied over the temperature range 50–70° at an ionic strength of 2·5M. In the acid range [H⁺]= 0·1–2·5M the increase in rate can be interpreted in terms of the reactions of the complex ions Co(αα-bipyridyl)₂(NO₂)₂⁺ and Co(αα-bipyridyl)₂(NO₂)(NO₂H)²⁺. Reaction rate constants have been obtained and thermodynamic parameters derived. The activation energies for the reactions of the complex ions are 24 ± 2 and 28 ± 2 kcal./mole, respectively. The trans–cis isomerisation of the nitroaquobis-(αα-bipyridyl)cobalt(III) ion has been studied in the range 0·2–2·5M-[H⁺] over the temperature range 56–70°. Rate data can be described in terms of relationships of the form k_obs=k_a/[H⁺]+k_b+k_c[H⁺], corresponding to the reactions of the ions trans-Co(αα-bipyridyl)₂(NO₂)(OH)⁺, trans-Co(α-bipyridyl)₂(NO₂)(H₂O)²⁺, and trans-Co(αα-bipyridyl)₂(NO₂H)(H₂O)³⁺. The corresponding activation energies are 41 ± 2, 33·5 ± 2, and 34·6 ± 2 kcal./mole, respectively. Mechanisms of the reactions are discussed.