We have studied the reactivity of a series of terminal alkenyldiphenylphosphines {Ph2P(CH2)n-CH═CH2), n = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co(I). Methane and ethane elimination occurs when different olefinic P,C-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co(CH3)(PMe3)4 with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic P,C,P-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe3)2Co{κ3-(P,C,P)-(Ph2)CH2CH═CH-CH(PPh2)} (1). X-ray study of 1 revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C(sp2)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co(CH3)(PMe3)4 afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe3)2Co{κ2(P,C)-HC═CHCH2PPh2} (2); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co(CH3)(PMe3)4 afforded a η3-coordinated π-allyl complex of composition (PMe3)2Co{η3-κ4-(P,C,C,C)-H2C═CH-CH(CH2)2PPh2} (3). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed.