Substitution Behaviour of an Alkylcobaltamine Complex − Evidence for a Limiting Dissociative Mechanism

Abstract

The substitution behaviour of pentaamminemethylcobalt(III) with ethylenediamine (en) was studied in detail in aqueous ammonia solution. The displacement of four ammonia ligands by two ethylenediamine (en) chelates was followed by a subsequent slow cis to trans rearrangement of the bis(ethylenediamine)(amine)methyl complex. The dependence of the substitution reaction on [en], [NH3], temperature and pressure was studied, and the observed kinetic traces 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 dissociative reaction mechanism under all conditions. For the first substitution reaction, ksat.(en)10 °C = 1.8 ± 0.4 s−1, ksat.(NH3)10 °C = 1.7 ± 0.3 s−1 and ΔV#15 °C = +14 ± 1 cm3·mol−1, for the second substitution reaction, ksat.(en)10 °C = 0.55 ± 0.12 s−1, ksat.(NH3)10 °C = 0.58 ± 0.10 s−1 and ΔV#15 °C = +24 ± 1 cm3·mol−1. The subsequent isomerization reaction was also studied in detail, and a complete set of activation parameters confirm the operation of a dissociative mechanism for the cis to trans rearrangement for which kiso25 °C = (1.48 ± 0.07) × 10−3 s−1; ΔH# = 115 ± 5 kJ·mol−1; ΔS# = + 86 ± 16 J·K−1·mol−1and ΔV# = +14.2 ± 0.6 cm3·mol−1.