The substituted sandwich complex crystallizes in monoclinic space group P21/m with Z = 2. Twinning to the (001) direction with the special condition c*/4a* = cos /5* causes systematic superposition of the reciprocal lattices of both domains and results in an apparent unit cell with double volume and the reflection condition (2h)kl, l = 2n. The structure solution was obtained with the subset of intensity data for the predominant individuum and converged at R = 0.040, Rw ---0.046 for 832 independent observations and 122 variables. The molecules show disorder with respect to the crystallographic mirror plane. The structure is closely related to that of deeamethylruthenocene.