The tetraethyl- and tetramethyl-cyclobutadiene complexes [(η4-C4R4)Co(η5-C5H4CHO)] R=Et, 5, R=Me, 7, and [(η4-C4R4)Co(η5-C5H4CO2Me)] R=Et, 6, R=Me, 8, are conveniently prepared by photolysis of the corresponding isocobaltocenium cations [(η4-C4R4)Co(η6-C6H5Me)]+ in acetonitrile, and subsequent treatment with Na[C5H4CHO] or Na[C5H4CO2Me]. The aldehydes 5 and 7 undergo Wittig and Knoevenagel reactions with [FcCH2PPh3]I and CH2(CN)2, to form [(η4-C4R4)Co(η5-C5H4CH=CHFc)] and [(η4-C4R4)Co(η5-C5H4CH=C(CN)2], 11 and 15, respectively. The Horner-Wittig reaction of [(η4-C4R4)Co(η5-C5H4CH2P(O)(OEt)2] with [(η4-C4Ph4)Co(η5-C5H4CHO)] yields [(η4-C4R4)Co(η5:η5-C5H4CHCH–C5H4)Co(η4-C4Ph4)], 12 and 13. [(η4-C4Me4)Co(η5-C5H4CHO)] also reacts with t-BuLi and FcLi to furnish the corresponding secondary alcohols, 16 and 17, respectively. Surprisingly, the attempted direct synthesis of 5 by reaction of Na[C5H5] and ethyl formate with [(η4-C4Et4)Co(CO)2I], 1, instead yielded [(η5-C5H5)Co(η4-3,4,5,6-tetraethyl-α-pyrone)], 18, and a mechanistic proposal is advanced. The X-ray crystal structures of 1, 7, 8, 11( Z ), 15 and 18, and also the isocobaltocenium salts [(η4-C4Et4)Co(η6-C6H5Me)][PF6], 2, and [(η4-C4Et4)Co(η6-1,3,5-C6H3Me3)][PF6], 4, are reported.