Water‐Exchange Mechanism of Tetraaquapalladium(II). A Variable‐Pressure and Variable‐Temperature Oxygen‐17 NMR Study

Water exchange of square‐planar Pd(H2O)24+ has been studied as a function of temperature (240 to 345 K) and pressure (0.1 to 260 MPa, at 324 K) by measuring the 17/O‐FT‐NMR line‐widths of the resonance from coordinated water at 27.11 and 48.78 MHz. The following exchange parameters were obtained: k298ex = (560 ± 40) s−1, ΔH* = (49.5 ± 1.9) kJ mol−1, ΔS* = – (26 ± 6) J K−1 mol−1 and ΔV* = – (2.2 ± 0.2) cm3 mol−1. The values refere to an aqueous perchlorate medium with an ionic strength between 2.0 and 2.6 m and a perchloric‐acid concentration between 0.8 and 1.7 m, and are interpreted in terms of an associative (a) activation for the exchange. The exchange rate for Pd(H2O)24+ is 1.4 × 106 times faster than for Pt(H2O)24+ at 298 K. A comparison with reactions between other nucleophiles and Pd(H2O)24+ is also made.