The substitution behavior of aquated Cu(II) ions is strongly affected by the presence of the tren (2,2’,2"-triaminotriethylamine) chelate. The kinetics and mechanism of solvent (water) exchange and of a series of complex-formation reactions of Cu(tren)H2O2+ were studied in detail at pressures up to 200 MPa using O-17 NMR and T-jump techniques. The water exchange rate on the pentacoordinate Cu(tren)H2O2+, k(ex)(298) = (2.39 +/- 10.09) x 10(6) s(-1), is over 3 orders of magnitude slower than on the hexacoordinate CU(H2O)(6)(2+). The reported volumes of activation strongly support the operation of an associative interchange (I-a) mechanism, viz. -4.7 +/- 0.2 cm(3) mol(-1) for water exchange and between -4.7 and -10 cm(3) mol(-1) for complex-formation and reverse aquation reactions involving pyridine and substituted pyridines. The latter results are used to construct volume profiles for the complex-formation reactions that further illustrate the volume collapse that occurs in going to the transition state for both reactions. The results are discussed with reference to earlier data reported in the literature. It is concluded that a chelate such as tren can induce a mechanistic changeover on labile aquated metal ions.