A family of terminal alkyne dicobalthexacarbonyl complexes bearing groups with a range of electron-withdrawing abilities has been synthesized. After submitting these complexes to the intermolecular Pauson–Khand reaction with norbornadiene, electron-deficient substrates afforded up to 26% of the unexpected endo-cyclopentenone. Since its discovery in 1971 the Pauson–Khand reaction (PKR) has become one of the most powerful tools for the synthesis of cyclopentenoid systems, both in its intra and intermolecular versions. 1 A wide array of biologi-cally significant natural products has been prepared by means of the PKR. 2 Our group and others have con-tributed to the development of highly efficient asym-metric versions of this process. 3 One of the key features that has led to the widespread use of the PKR is its high degree of regio and stereo-selectivity. Early on, for the intermolecular version of the PKR, it was demonstrated that the cycloaddition reaction took place invariably at the less hindered face of the olefin. Reaction with norbornene, norbornadiene and bicyclo[3.2.0]heptene yields exclusively the exo-fused cyclopentenone adducts (Scheme 1). 1a