Formation of η³-bonded lactone complexes and eight-membered ring metallacycles with ketene groups by the insertion of acetylenes into carbon–molybdenum and –tungsten σ bonds; molecular and crystal structures of carbonyl(η-cyclopentadienyl)(3–5-η-2,3-dihydro-3,4,5-trimethyl-2-oxo-3-furyl)(t-butyl isocyanide)molybdenum and carbonyl-(η-cyclopentadienyl)(3–4:5–6-η-2,3,4,5-tetramethyl-6-oxo-1-trifluoromethylhexa-1,3,5-trienyloxo)molybdenum

Reaction of [MR(CO)₃(η-C₅H₅)](M = Mo, R = COCF₃, Me, or PhCH₂; M = W, R = COCF₃ or Me) with but-2-yne either thermally or on u.v. irradiation affords the crystalline vinylketone complexes [[graphic omitted]}-(CO)₂(η-C₅H₅)](R = CF₃, Me, or PhCH₂). These complexes react with donor ligands such as CNBu^t, CNC₆H₁₁, CO, or PPh₃ to form η³-allylic lactone complexes of the type [M{η³-[graphic omitted]}(CO)(η-C₅H₅)L](L = CO, CNBu^t, CNC₆H₁₁, or PPh₃). The complex where L = CNBu^t and R = Me is structurally identified by single-crystal X-ray diffraction. Crystals are monoclinic, space group P2₁/a with four molecules in a unit cell of dimensions a= 9.3013(15), b= 22.161(6), c= 9.353(3)Å, and β= 102.76(2)°. The mode of formation of this type of complex is discussed in detail. A further product obtained from the reaction of [Mo(COCF₃)(CO)₃-(η-C₅H₅)] with but-2-yne is the purple-red complex [[graphic omitted]}(CO)(η-C₅H₅)] crystallographically shown to contain an unusual eight-membered ring structure containing an η²-ketenic group. Eight molecules crystallise in a unit cell of dimensions a= 18.389(4), b= 11.137(3), c= 16.686(5)Å, β= 92.55°, and space group P2/n. The mechanism of formation of this and of a related tungsten complex is discussed. In addition, complexes are formed which are considered to be isostructural with the eight-membered ring species, except that the terminal carbon monoxide is replaced by an η²-bonded but-2-yne molecule. Simple symmetry arguments are developed which refer the co-ordination geometries of both the lactone and the metallacycle complexes to different isomers of a monocapped trigonal prism.