Reaction of Grignard reagents MgRX or alternatively MgX₂ with [Mo(η²-R′C₂R″)₂(CO)L][BF₄](R′= R″= Me; R′= Me, R″= Ph; R′= Bu^t, R″= H; L =η-C₅H₅ or η⁵-C₉H₇) afforded the air-sensitive complexes [MoBr(η²-MeC₂Me)(CO)(η-C₅H₅)]2, [MoCl(η²-MeC₂Me)(CO)(η-C₅H₅)]3, [MoI(η²-MeC₂Me)(CO)(η-C₅H₅)]4, [MoBr(η²-MeC₂Me)(CO)(η⁵-C₉H₇)]5, [MoBr(η²-Bu^tC₂H)(CO)(η-C₅H₅)]6 and [MoBr(η²-PhC₂Me)(CO)(η-C₅H₅)]7. Reaction of 2 with isoprene and AgBF₄ leads to a novel carbon–carbon coupling reaction and the formation of [Mo{η³,η³-CH(Me)C(Me)CHCHC(Me)CH₂}(CO)(η-C₅H₅)][BF₄]8, the single-crystal X-ray diffraction structure of which shows the presence of an unusual η³,η³-bonded hexatrienyl ligand which wraps itself around a Mo(CO)(η-C₅H₅)⁺ fragment. Generalisation of the reaction established that buta-1,3-diene undergoes analogous reactions and that unsymmetrical alkynes couple with the 1,3-diene regioselectively. Reaction of 7 with CD₂CHCHCD₂ and AgBF₄ afforded [Mo{η³,η³-CD(Ph)C(Me)CDCHCHCD₂}(CO)(η-C₅H₅)][BF₄] leading to definitive conclusions about the mechanism of the coupling reaction. The reaction of 8 with nucleophilic reagents has been examined, Li[BHEt₃], LiMe and LiPh delivering ‘H^–’, ‘Me^–’ and ‘Ph^–’ to the substituted end carbon of the η³,η³-C₆ ligand, whereas the sterically more demanding reagent K[BHBu^s₃] delivers ‘H^–’ to both ‘inside’ and ‘outside’ carbons. Reaction of 2, [MoCl(η²-PhC₂Ph)(CO)(η-C₅H₅)] and 5 with C₂H₄ and AgBF₄ affords the alkene–alkyne complexes [Mo(η²-C₂H₄)(η²-MeC₂Me)(CO)(η-C₅H₅)][BF₄], [Mo(η²-C₂H₄)(η²-PhC₂Ph)(CO)(η-C₅H₅)][BF₄] and [Mo(η²-C₂H₄)(η²-MeC₂Me)(CO)(η⁵-C₉H₇)][BF₄]. The reactions of the first of these products with nucleophilic reagents, P(OMe)₃, CO and isoprene, have been studied.