Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

The mechanism of the Pauson–Khand reaction has attracted significant interest due to the unusual dinuclear nature of the Co 2 (CO) x active site. Experimental and computational data have indicated that the intermediates following the initial Co 2 (CO) 6 (alkyne) complex are thermodynamically unstable and do not build up in appreciable concentrations during the course of the reaction. As a consequence, the key steps that control the scope of viable substrates and various aspects of selectivity have remained largely uncharacterized. Herein, a direct experimental investigation of the dinuclear metallacycle‐forming step of the Pauson–Khand reaction is reported. These studies capitalize on well‐defined d 9 –d 9 dinickel complexes supported by a naphthyridine–diimine (NDI) pincer ligand as functional surrogates of Co 2 (CO) 8 .