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 ma de at variable pressure with a le Noble-Schlott (''pillbox'') cell for the inner sleeve wall thickness. With this technique, the molar volum e change for the acid ionization of aqueous [Cr(Hedta)OH2] was found t o be +5.1 +/- 0.6 cm(3) mol(-1) (0-200 MPa, 25.0 degrees C, ionic stre ngth 1.0 mol L(-1) HClO4/NaClO4), an anomalous positive value which im plies a change from quinquedentate to predominantly sexidentate edta a nd expulsion of the coordinated water on ionization. For thiocyanate s ubstitution into labile [Cr(Hedta)OH2], high pressure stopped-now meas urements gave the volume of activation as -7.8 +/- 0.9 cm(3) mol(-1) a nd the volume of reaction as +3 +/- 2 cm(3) mol(-1), while for the rea ction of [Cr(edta)](-) with NCS- the activation volume is -13.6 +/- 0. 6 cm(3) mol(-1) (same conditions). These and other data support the no tion that the anomalous substitutional lability of Cr-III(edta) comple xes relative to typical Cr-III species is due to activation by transie nt chelation of the pendant arm of quinquedentate edta.