Substitution behaviour of an alkylcobaltamine complex - Evidence for a limiting dissociative mechanism

The substitution behaviour of pentaamminemethylcobalt(III) with ethylenedia mine (en) was studied in detail in aqueous ammonia solution. The displaceme nt of four ammonia ligands by two ethylenediamine ten) chelates was followe d by a subsequent slow cis to trans rearrangement of the bis(ethylenediamin e)(amine)methyl complex. The dependence of the substitution reaction on [en ], [NH3], temperature and pres sure was studied, and the observed kinetic t races could be fitted to the sum of two exponentials. The rate law for both reaction steps showed saturation kinetics with respect to [en] and [NH3], and the activation parameters confirmed the operation of a limiting dissoci ative reaction mechanism under all conditions. For the first substitution r eaction, k(sat.)(en)(10 degrees C) = 1.8 +/- 0.4 s(-1), k(sat.)(NH3)(10 deg rees C) = 1.7 +/- 0.3 s(-1) and Delta V-15 degrees C(#) = +14 +/- 1 cm(3).m ol(-1), for the second substitution reaction, k(sat.)(en)(10 degrees C) = 0 .55 +/- 0.12 s(-1), k(sat.)(NH3)(10 degrees C) = 0.58 +/- 0.10 s(-1) and De lta V-15 degrees C(#) = +24 +/- 1 cm(3).mol(-1). The subsequent isomerizati on reaction was also studied in detail, and a complete set of activation pa rameters confirm the operation of a dissociative mechanism for the cis to t rans rearrangement for which k(iso25 degrees C) = (1.48 +/- 0.07) x 10(-3) s(-1); Delta H-# = 115 +/- 5 kJ.mol(-1); Delta S-# = + 86 +/- 16 J.K-1.mol( -1) and Delta V-# = +14.2 +/- 0.6 cm(3).mol(-1).