Norbornadiene and conjugate dienes react with Co₂(CO)₈ to give {(π-diene)Co₂(CO)₆}(I) and then {(π-diene)-Co(CO)₂}₂(II)(diene = norbornadiene, isoprene, and 2,3-dimethylbuta-1,3-diene). The hexacarbonyl derivatives (I) are not tautomeric, and have structures based on that of the bridged isomer of octacarbonyldicobalt. Both (I) and (II) react with iodine to give {(π-diene)Co(CO)₂I} derivatives which could be detected but not isolated, with SnX₄ to give mixtures of {(π-diene)Co(CO)₂SnX₃} and {Co(CO)₄SnX₃}(X = Cl, Br, or I), and with Ph₃PAuCl, HgX₂, and Ph₃SnCl to give only Ph₃PAuCo(CO)₄, Hg{Co(CO)₄}₂, and Ph₃SnCo(CO)₄ respectively. Monodentate tertiary phosphines convert (I) to a mixture of (II) and {(R₃P)Co(CO)₃}₂via the unstable intermediate ionic species [(π-diene)Co(CO)₂PR₃][Co(CO)₄]. Similar salts are stable and have been isolated with Ph₂PCH₂CH₂PPh₂i.e.[{(π-diene)Co(CO)₂}₂{Ph₂PCH₂CH₂PPh₂}][Co(CO)₄]₂. They are reduced by NaBH₄ to {(π-allyl)Co(CO)₂}₂{Ph₂PCH₂CH₂PPh₂}. The i.r. spectra of the products are reported and their structures discussed. The reactions are compared, and contrasted, with those of the related isoelectronic and isostructural π-cyclopentadienyl complexes of iron.