Variable temperature and pressure study of the aquation reactions of cobalt(III) and chromium(III) penta- and tetra-amines†

Abstract

Preparation of a series of specific penta- and tetra-amine derivatives of Co^III and Cr^III with a neutral leaving ligand has been carried out in order to accomplish a fine tuning of the associativeness/dissociativeness of their substitution reactions. Spontaneous aquation reactions of the neutral ligands have been studied at variable temperature and pressure. Although rate constants and thermal activation parameters show an important degree of scatter, the values determined for the activation volumes of the substitution process illustrate the mechanistic fine tuning that may be achieved for these reactions. In all cases, in the absence of important steric constraints in the molecule, electronic inductive effects seem to be the most important factor accounting for the dissociative shifts observed both for pentaamine (i.e. ΔV^‡ = +4.0 or +14.0 cm³ mol^–1 and +5.2 or +16.5 cm³ mol^–1 for the aquation of cis- or trans-[Co(MeNH₂)(NH₃)₄(DMF)]³⁺ and cis- or trans-[CoL₁₅(DMF)]³⁺ respectively, where L₁₅ represents a pentaamine macrocyclic ligand), and tetraamine systems (i.e. ΔV ^‡ = +4.1 or +8.4 cm³ mol^–1 and –10.8 or –7.4 cm³ mol^–1 for the aquation of cis-[Co(NH₃)₄Cl(DMAC)]²⁺ (DMAC = dimethylacetamide) or cis-[Co(en)₂Cl(DMAC)]²⁺ and cis-[Cr(NH₃)₄Cl(DMF)]²⁺ or cis-[Cr(en)₂Cl(DMF)]²⁺). From the results, clear evidence is obtained which indicates that, only when the situation is borderline I_a/I_d, or the steric demands are increased dramatically, dissociative shifts are observed; in all other cases electronic inductive effects seem to be dominant for such a tuning of the substitution process.