The reaction of RuCl 3(H 2O), with C 5Me 4CF 3J in refluxing EtOH gives [Ru 2( η 5-C 5Me 1CF 2) 2 ( μ-Cl 2] ( 20 in 44% yield. Dimer 2 antiferromagnetic (−2 J200 cm 1). The crystal structures of 2 (rhombohedral system, R3 space group, Z=9, R=0.0589) and [Rh 2( η 5-C 5Me 4CF 3( 2Cl 2( μ-Cl) 2] (3) (rhombohedral system. R 3 space group, Z = 9, R = 0.0641) were solved; both complexes have dimeric structures with a trans arrangement of the η 5-C 5Me 4CF 4 rings. Comparison of the geometry of 2 and 3 with those of the corresponding η 5-C 5Me 5 complexes shows that lowering the ring symmetry causes significant distortion of the M 2(μ-Cl) 2 moiety. The analysis of the MCl 3 fragment conformations in 2 and 3 and in the η 5-C 5ME 5 analogues shows that they are correlated with the MM distances. The Cl atoms are displaced by Br on reaction of 2 with KBr in MeOH to give the diamagnetic dimer [Ru 2( η 5-C 5Me 4CF 3) 2Br 2 ( μ-Br 2] ( 4). Complex 2 reacts with O 2 in CH 2Cl 2 solution at ambient temperature to form a mixture of isomeric η 6-fulvene dimers [Ru 2( η 6-C 5Me 3CF 3 = CH 2) 2Cl 2( μ-Cl) 2] ( 5). Reactions of 5 with CO and allyl chloride give Ru( η 5-C 5Me 3CF 3CH 2Cl)(CO) 2Cl ( 6) and Ru( η 5-C 5Me 3CF 3CF 3CH 2Cl)( η 3-C 3H 5)Cl 2 ( 7) respectively.