Group 9 bimetallic carbonyl permethylpentalene complexes.

We describe the synthesis, structure and bonding of the first iridium and rhodium permethylpentalene complexes, syn-[M(CO)2]2(μ:η(5):η(5)-Pn*) (M = Rh, Ir). In fact, [Ir(CO)2]2(μ:η(5):η(5)-Pn*) is the first iridium pentalene complex. An interesting preference for the isolation of the sterically more demanding syn-isomer is observed and substantiated by DFT analysis. Upon photolysis, the rhodium analogue yields an unusual tetrameric species Rh4(CO)6(μ:η(3):η(5)-Pn*)2 with bridging carbonyls and Rh-Rh bonds, which has been characterised by single crystal X-ray diffraction and by solution NMR spectroscopy.