(π-allyl)Ni complex, which readily cyclises in a “metallo-ene”-type reaction. The different pathways were characterised by further carbonylinsertion or by protonolysis, resulting either in cyclopentanone formation (insertion followed by β-elimination) when under carbonmonoxide gas, or in single cyclization when not (or, even better, when in the presence of protic acids). Coordination of the distal olefin