High‐Pressure 17 O‐NMR. Study of Vanadium (II) in Water: A Second Example of an Associative Interchange Mechanism (I a ) for Solvent Exchange on an Octahedral Divalent Transition‐Metal Ion

The water exchange of [V(H 2 O) 6 ]Cl 2 in aqueous solution has been studied as a function of temperature and pressure (up to 250 MPa), by measuring the 17 O‐FT‐NMR. line‐widths of the free water resonance at 8.13 MHz. The kinetic parameters obtained are K   298 m= 87±4 s −1 , ΔH * = +61.8 ± 0.7 kJ mo1 −1 and ΔS * = −0.4±1.9 J mol −1 K −1 . A pressure‐independent volume of activation ΔV * = −4.1±0.1 cm 3 mol −1 is obtained, suggesting an associative interchange (I a ) mechanism for this early divalent metal ion.