Volume Profile for Substitution in Labile Chromium(III) Complexes:  Reactions of Aqueous [Cr(Hedta)OH2] and [Cr(edta)]- with Thiocyanate Ion

A procedure is given for correcting optical absorbance measurements made at variable pressure with a le Noble−Schlott (“pillbox”) cell for the inner sleeve wall thickness. With this technique, the molar volume change for the acid ionization of aqueous [Cr(Hedta)OH2] was found to be +5.1 ± 0.6 cm3 mol-1 (0−200 MPa, 25.0 °C, ionic strength 1.0 mol L-1 HClO4/NaClO4), an anomalous positive value which implies a change from quinquedentate to predominantly sexidentate edta and expulsion of the coordinated water on ionization. For thiocyanate substitution into labile [Cr(Hedta)OH2], high pressure stopped-flow measurements gave the volume of activation as −7.8 ± 0.9 cm3 mol-1 and the volume of reaction as +3 ± 2 cm3 mol-1, while for the reaction of [Cr(edta)]- with NCS- the activation volume is −13.6 ± 0.6 cm3 mol-1 (same conditions). These and other data support the notion that the anomalous substitutional lability of CrIII(edta) complexes relative to typical CrIII species is due to activation by transient chelation of the pendant arm of quinquedentate edta.