Kinetics and mechanism of the base hydrolysis of cis-eq-[Cr(NCS)(S-pdtra)]−, trans-eq-[Cr(NCS)(edtrp)]− and trans-eq-[Cr(NCS)(R-pdtrp)]− complexes. Strong rate enhancement for NCS− ligand release with an increase of [OH−] only for the trans-equatorial isomers

The [Cr(NCS)(edtrp)]−, [Cr(NCS)(R-pdtrp)]− and [Cr(NCS)(S-pdtra)]− complexes, that are derivatives of the trans-equatorial isomers of [Cr(edtrp)(H2O)]° and [Cr(R-pdtrp)(H2O)]° and the cis-equatorial isomer of [Cr(S-pdtra)-(H2O)]° (edtrp = ethylenediamine-N,N,N′-tripropionate, R-pdtrp = R-propane-1,2-diamine-N,N,N′-tripropionate, S-pdtra = S-propane-1,2-diamine-N,N,N′-triacetate) undergo aquation in alkaline media with a strong dependence of the rate on [OH−] for the trans-equatorial isomers and a very weak dependence for the cis-equatorial isomer. The thiocyanate ligand release follows a stereoretentive course for all reactants. Based on kinetic data the reaction mechanism has been discussed. Rate differences between the isomers are interpreted in terms of an interchange via a conjugate base (I c.b.) mechanism, assuming an equilibrium between the cis-equatorial-CrIII-S-pdtra complexes with penta- and tetradentate coordination of the edta-like ligand.