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Water exchange on the light lanthanide aqua ions [Pr(H2O)9]3+ and [Nd(H2O)9]3 +: A variable temperature and magnetic field 17O NMR study

17O NMR relaxation rate measurements were made on aqueous perchlorate solutions of the light lanthanide ions Nd3+ and Pr3+, with the aim of measuring the kinetics of water exchange on these ennea aqua ions. Variable‐temperature results at three magnetic fields show that the ‘kinetic’ effect is not the only contribution to the difference between the transverse and longitudinal relaxation rates. The results at high magnetic field allow lower limits, k  ex298 ⩾ 5 × 108 S−1 and k  ex298 ⩾ 4 × 108 s−1, to be determined for water exchange rates at 298.15 K on [Nd(H2O)9]3+ and [Pr(H2O)9]3+, respectively. These limits are consistent with our current understanding of water exchange on the lanthanide aqua ions, based on the change of relative stability of the ennea and octa aqua ions across the series.