The reactions of HRu3(CO)9C2C(CH3)3 with 2cis, 4trans-hexadiene and 1,5-hexadiene afford as main product the same complex HRu3(CO)7(C6H9)(C6H10)suggesting that isomerization of the non conjugated species occurs in the reactiotion. The crystal structure of this compound has been determined by X-ray methods. Crystals are triclinic, space group P1, with Z = 2, in a unit cell of dimensions a = 10.230(8), b = 10.487(9), c = 19.084(13) Å α = 49.3(1), ß = 126.4(1), and γ = 127.9(1)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.054 for 4303 observed reflections. The molecule is formed by a three-metal atom cluster with seven terminal CO groups; one hydridic hydrogen atom is bridge-bonding two ruthenium atoms on one side of the cluster. A t-butylethynyl group it bound to the three metals via a σ bond to one Ru and two η bonds to the other two Ru atoms; opposite to this group with respect to the cluster a 2trans, 4trans-hexadienic unit bridges two Ru atoms via two η bonds. © 1979.