Dynamics and reactivity of µ(1-σ,5,6-η:2–4-η-cycloheptadienyl)- and µ-(1-σ,5,6-η:2–4-η-cyclo-octadienyl)-bis[(η-cyclopentadienyl)rhodium](Rh–Rh)

Abstract

The title complexes, (I) and (II), have been prepared and characterised. Both adopt the 1 -σ,5,6-η:2–4-η-structure in solution. High-temperature ¹H and ¹³C n.m.r. spectra indicate that in each system there is an apparent mirror plane in the dienyl ring and that the two η-cyclopentadienyl ligands in each complex remain distinct. This is achieved by oscillation of the metal–metal axis with respect to the dienyl ring. When treated with strong acid both complexes (I) and (II) form cationic complexes containing bridging hydride ligands. For the cyclo-octadienyl complex, (I), this also causes a marked increase in the energy barrier to the oscillation; for (II) the bonding to the dienyl changes to the 1–3-η:4–6-η-type. Using CF₃CO₂D, the added deuteron is incorporated entirely in a methylene group in complex (I), and 50% so in (II). The displaced proton forms the bridging hydride ligand. The effect of the dienyl ligand on the metal–metal bond is considered and these systems are compared with corresponding bis(tricarbonyliron) systems.