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′= But, 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-ButC2H)(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[BHBus3] 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.