Crystals of the title compound (I) are orthorhombic, space group Pnma, a= 17·54(3), b= 14·55(2), c= 6·766(10)Å, Z= 4. The structure was solved by the heavy-atom method and refined by full-matrix least-squares to R 0·047 for 1018 unique reflections measured by diffractometer. The molecule lies on a crystallographic mirror plane and possesses approximately C3ν(3m) symmetry; principal bond lengths (averaged for C3ν): Ru ⋯ Ru 2·842(6), Ru–C(O) 1·869(15) and 1·956(19), Ru–C(C) 2·083(11), C–O 1·152(16), and C–C 1·511(20)Å. The bridging hydrogen atoms were located in symmetrical positions on the opposite side of the Ru3 plane from the CMe moiety; their positions were refined from the X-ray data subject to the constraints that all Ru–H and all (C)C–H distances were respectively equal, leading to Ru–H, 1·72(7)Å and Ru–H–Ru, 112(7)°. The nematic-phase proton n.m.r. spectrum has been reinterpreted in the light of the idealised heavy-atom geometry found in the crystal, and used to give calculated values for Ru–H (1·81 Å) and Ru–H–Ru (103°), which are probably more accurate than those calculated only from X-ray data.