The relative nucleophilic efficiency of water and dimethyl sulphoxide in the aqueous substitution of aquapentacyanocobaltate(III) and chloropentacyanocobaltate(III)

Abstract

The kinetics of substitution of [Co(CN)₅Cl]^3– by water and by dimethyl sulphoxide (dmso) may be quantitatively interpreted by the mechanism in equations (i)–(iii). High-performance liquid [CO(CN)₅Cl]^3–+ H₂O [graphic omitted] [Co(CN)₅(OH₂)]^2–+ Cl^–(i), [Co(CN)₅Cl]^3–+ dmso [graphic omitted] [Co(CN)₅(dmso)]^2–+ Cl^–(ii), [Co(CN)₅(OH₂)]^2–+ dmso [graphic omitted] [Co(CN)₅(dmso)]^2–+ H₂O (iii) chromatographic analysis confirms the direct formation of a single, probably sulphur-bonded, isomer of [Co(CN)₅(dmso)]^2– from [Co(CN)₅Cl]^3– as well as via[Co(CN)₅(OH₂)]^2–. The kinetics suggests the participation of the dissociatively formed intermediate [Co(CN)₅]^2–, but charged nucleophiles (NCS^– or N₃^–) react more slowly with [Co(CN)₅]^2– than do the uncharged nucleophiles (dmso and water) when the leaving group is Cl^– compared to the case when the leaving group is OH₂. These results support the view that the substitution occurs by an I_d interchange mechanism.