[Fe2(CO)6(μ-CO)(μ-C(C6H5)C(C6H5)H)]- was reacted with RC≡CR aikynes (R = CH3OC(O), CF3) affording [Fe2(CO)6(μ-C(R)C(R)C(O)C(C6H5)C(C6H5)H)]- (R = CH3OC(O) (3), CF3 (5)) and [Fe2(CO)6(μ-C(C6H5)-C(C6H5)C(R)C(R)H)]- (R = CF3 (4)). The structure of 3 and 4 has been determined by single-crystal X-ray crystallography. Complex 3 crystallizes in the space group P21/n with Z = 4 and lattice constants a = 17.244 (6) Å,b = 23.419 (7) Å, c = 12.218 (5) Å, and β = 94.31 (4)°. This product results from the double insertion of carbon monoxide and alkyne into the Fe-C bond of the ethenyl ligand. Furthermore, the oxygen of the acyl group is bonded to a Fe atom. Complex 4 crystallizes in the space group P1 with Z = 2 and lattice constants a = 15.397 (5) Å,b = 14.495 (4) Å, c = 11.258 (4) Å, α = 107.35 (3)°, β = 96.92 (2)°, and γ = 110.85 (3)°. This product results from the insertion of the alkyne into the carbon-hydrogen bond of the ethenyl ligand. Furthermore, the C(CF3)C(CF3)H fragment of the organic part of the molecule is μ-ƞ1 bonded to the two iron atoms. A possible mechanism for the two types of insertion observed is proposed. © 1984, American Chemical Society. All rights reserved.