The stable salts [RuH(diene)L3][PF6]{1; diene = cyclo-octa-1,5-diene (cod), hexa-1,3-diene, or buta-1,3-diene, L = PMe2Ph; diene = cod, L = P(OMe)3, P[(OCH2)3CMe], PPh2(OMe), PMe3, or PMePh2} have been prepared from the salt [RuH(cod)(NH2NMe2)3][PF6], and have been characterized by i.r., microanalytical, and 1H n.m.r. measurements. The crystal structure of [RuH(C4H6)(PMe2Ph)3][PF6]·ca. 0.5 CH2Cl2 has been determined. The yellow crystals are monoclinic, space group P21/n, a= 17.85, b= 18.96, c= 10.31 A, β= 106.1°. The structure has been solved by the heavy-atom method and refined by least squares to R 0.070 for 2 429 observed data collected on a diffractometer using graphite-monochromatized Mo-Kα radiation. The ruthenium has a distorted-octahedral co-ordination geometry. It is bonded to all the carbon atoms of the butadiene molecule; Ru–C 2.14–2.38 A. The phosphine ligands have a fac configuration; Ru–P 2.28–2.34 A, P–Ru–P 94–100°. The butadiene moiety is approximately planar, with bond lengths of 1.48, 1.34, and 1.43 A(estimated standard deviation 0.03 A). The position of the hydride ligand is not directly observable but can be inferred from the arrangement of the other ligands in the cation.