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

Water exchange of square‐planar Pd(H 2 O) 2 4 + 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: k 298 ex = (560 ± 40) s −1 , Δ H * = (49.5 ± 1.9) kJ mol −1 , Δ S * = – (26 ± 6) J K −1 mol −1 and Δ V * = – (2.2 ± 0.2) cm 3 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(H 2 O) 2 4 + is 1.4 × 10 6 times faster than for Pt(H 2 O) 2 4 + at 298 K. A comparison with reactions between other nucleophiles and Pd(H 2 O) 2 4 + is also made.