Kinetics and mechanisms of the reactions of nickel(II) and pentane-2,4-dione

The kinetics and mechanisms of the reactions of Ni^II and pentane-2,4-dione (Hpd) to form the mono complex have been investigated in water and aqueous methanol (mol fraction of methanol, 0.8). The keto tautomer does not react directly with Ni^II, and the kinetic data are consistent with a mechanism in which Ni^II reacts with the enol form of the ligand. The rate constants for reaction with the undissociated enol tautomer are 19.3 dm³ mol^-1 s^-1 in water and 17.0 dm³ mol^-1 s^-1 in aqueous methanol. The results suggest that, for the reaction of metal and oxometal ions with simple β-diketones, a mechanism whereby complex formation rates are closely related to solvent exchange rates is not operative. The results are discussed and rationalized in terms of previously published data for reactions of Cu^II, Fe^III, and U^VI with β-diketones.