Abstract The kinetics and mechanism of intramolecular hydroacylation catalyzed by complexes Co 2(μ-N 2)(PPh 3) 6 (III) and (PPh 3) 2Co(dppe) (IV) (dppe-1,2-bis(di-phenylphosphino)ethane) were studied. The active monomeric 17-electron intermediate, (Ph 3P) 2Co(CH 2CHCH 2CH 2CHO) (Vb) was isolated in the cyclisation of 4-pentenal (I), catalyzed by complex III. Crystals of Vb are stable in an inert atmosphere at room temperature for a month. A well defined EPR spectrum of Vb in toluene glass at 77 K was obtained. According to the IR spectrum of complex Vb aldehyde I is bonded to the metal as a bidentate π,π-ligand. The ligand exchange reaction was found to proceed readily between Vb and dppe to form free I (85%) and Co(dppe) 2. The complex Vb is the first example of a cobalt(0) complex with a coordinated aidehyde group. The analogous intermediate, (dppe)Co(CH 2CHCH 2-CH 2CHO) (Va) was identified by EPR in solution during the 4-pentenal cyclization, catalyzed by complex IV. For the 4-pentenal hydroacylation reaction, catalyzed by complex Vb, zero and first order kinetic laws were found for the aldehyde I and the catalyst concentrations respectively in accordance with the equation: W 0 = k[Vb], k = (8.5 ± 0.5) × 10 −4 s −1 at 70°C. It is suggested that the reaction mechanism involves oxidative addition of coordinated I to cobalt(0) within the complex Vb to give an acyl hydride intermediate (rate-determining step). The subsequent rearrangement of the acyl hydride complex to the acylalkylcobalt derivative, followed by a reductive elimination reaction yields the ketone II and the parent cobalt(0) complex. Unlike well-known catalytic processes, in which diamagnetic Co I and Co III species are key intermediates, the catalytic cycle of the intramolecular hydroacylation reaction seems to involve only mononuclear paramagnetic Co 0 and Co II species. Chelation of the cobalt atom by an unsaturated aldehyde was found to be an important requirement for this reaction to occur.