Cyclic Enolates of Ni and Pd: Equilibrium between C‐ and O‐Bound Tautomers and Reactivity Studies

2‐Acylaryl complexes of Ni and Pd containing chelating diphosphines react with K t BuO to give metallacyclic enolate complexes. While coordination through the carbon atom is preferred in the case of Pd, the nickel O ‐enolate compounds are formed as the corresponding O‐tautomers. Slow equilibration between O ‐ and C ‐enolate tautomers is observed for the nickel complex with an unsubstituted enolate function (MOCCH 2 ). Theoretical DFT calculations suggest that the barrier for the tautomer exchange has its origin in the rigidity of the metallacycle. Whilst the C ‐enolate tautomer is unreactive towards aldehydes, the corresponding O ‐enolate adds to MeCHO and PhCHO, giving rise to products that retain the enolate functionality. The carbonylation of these products cleanly leads to the formation of enol lactones in a highly selective manner.