The influence of molecular hydrogen on the title reactions has been studied; different products (and yields) are obtained when this gas is used, instead of nitrogen, as reaction atmosphere. From the reactions of [Fe₃(CO)₁₂] with [Ni₂(η-C₅H₅)₂{HC₂C(CH₂)CH₃}] the new heterometallic complex [Ni₂Fe₂(η-C₅H₅)₂(CO)₆{C₂(H)C(CH₂)CH₃}] is obtained, whose structure has been determined by X-ray methods. Crystals are triclinic, space group P, with a unit cell of dimensions a= 8.881(7), b= 17.224(12), c= 7.912(7)Å, α= 78.87(4), β= 114.95(6), γ= 91.60(4)°, and Z= 2. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.046 for 2 965 observed reflections. The complex shows a ‘spiked-triangular’ metal atom frame and the alkyne ligand interacts with all four metals through both triple and double bonds. An isomeric complex has been also characterized and a square-planar structure has been assigned to it on the basis of spectroscopy and by comparison with known compounds. From, the reactions of [Ru₃(CO)₁₂] with the same alkyne, instead of the expected [NiRu₃(η-C₅H₅)(µ-H)(CO)₉{CC(H)C(CH₂)CH₃}], the already known [NiRu₃(η-C₅H₅)(η-H)(CO)₉{CC(H)C(H)(CH₃)₂}] complex has been obtained, by consequence of the hydrogenation of the reactant double bond.