Abstract The tetraethyl- and tetramethyl-cyclobutadiene complexes [(η 4-C 4R 4)Co(η 5-C 5H 4CHO)] R = Et, 5, R = Me, 7, and [(η 4-C 4R 4)Co(η 5-C 5H 4CO 2Me)] R = Et, 6, R = Me, 8, are conveniently prepared by photolysis of the corresponding isocobaltocenium cations [(η 4-C 4R 4)Co(η 6-C 6H 5Me)] + in acetonitrile, and subsequent treatment with Na[C 5H 4CHO] or Na[C 5H 4CO 2Me]. The aldehydes 5 and 7 undergo Wittig and Knoevenagel reactions with [FcCH 2PPh 3]I and CH 2(CN) 2, to form [(η 4-C 4R 4)Co(η 5-C 5H 4CH=CHFc)] and [(η 4-C 4R 4)Co(η 5-C 5H 4CH=C(CN) 2], 11 and 15, respectively. The Horner-Wittig reaction of [(η 4-C 4R 4)Co(η 5-C 5H 4CH 2P( O)(OEt) 2] with [(η 4-C 4Ph 4)Co(η 5-C 5H 4CHO)] yields [(η 4-C 4R 4)Co(η 5:η 5-C 5H 4CH CH–C 5H 4)Co(η 4-C 4Ph 4)], 12 and 13. [(η 4-C 4Me 4)Co(η 5-C 5H 4CHO)] 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[C 5H 5] and ethyl formate with [(η 4-C 4Et 4)Co(CO) 2I], 1, instead yielded [(η 5-C 5H 5)Co(η 4-3,4,5,6-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 [(η 4-C 4Et 4)Co(η 6-C 6H 5Me)][PF 6], 2, and [(η 4-C 4Et 4)Co(η 6-1,3,5-C 6H 3Me 3)][PF 6], 4, are reported.