Reaction of [Mo(CO)(η2-MeC2Me)2(η-C5H5)][BF4] with lithium dimethylcuprate affords [Mo(CO)(η2-MeC2Me)(σ-CMeCMe2)(η-C5H5)], whereas reaction with K[BHBus3] followed by treatment with CO gives [[graphic omitted])C(Me)CHMe}(CO)2(η-C5H5)], which has been identified by X-ray crystallography. Crystals are monoclinic, space group P21/n, Z= 4, in a unit cell with a= 6.495(3), b= 16.902(9), c= 14.002(7)A, β= 97.62(4)°. The structure has been refined to R= 0.025 (R′= 0.024) for 2 136 reflections (293 K) to 2θ < 55°(Mo-KαX-radiation). The molybdenum atom is co-ordinated by an η-C5H5 ligand, two terminal linear carbonyl ligands, and a bidentate ligand derived from one CO, two but-2-yne molecules, and a hydride ligand. Reaction of CO with [Mo(CO)(η2-MeC2Me)(σ-CMeCMe2)(η-C5H5)] affords an analogous complex. A similar reaction between K[BHBus3] and [Mo(CO)(η2-EtC2Me)2(η-C5H5)][BF4] gives two analogous isomeric complexes, both isomers containing the vinyl group, C(Me)CHEt. In contrast [Mo(CO)(η2-ButC2H)2(η-C5H5)][BF4] affords the lactone complex [Mo{η3:η2-HButCCH[graphic omitted]H}(CO)(η-C5H5)] characterised by X-ray crystallography. Crystals are monoclinic, space group P21/c(no. 14), Z= 4, in a unit cell with a= 11.935(7), b= 11.673(9), c= 13.674(9)A, β= 105.93(5)°. The structure has been refined to R= 0.053 (R′= 0.052) for 2 526 reflections (230 K) to 2θ⩽ 50°(Mo-KαX-radiation). The molybdenum atom is bound to an η-cyclopentadienyl ligand and a linear terminal CO. The remainder of the co-ordination sphere is taken up by an η3 : η2 vinyl-substituted, unsaturated γ-lactone derived from one hydride, two carbonyls, and two ButC2H ligands. The structures of these complexes and the mechanism of their formation are discussed.