Abstract
The redox condensation of [Ir(CO)~4~]^–^, [Ir(cod)(THF)~2~]^+^, and [Rh(cod)(THF)~2~]^+^ (cod = cycloocta‐1,5‐diene) followed by saturation with CO (1 atm) in THF afforded the first synthetic route to pure [Ir~3~Rh(CO)~12~] (1). Substitution of CO by monodentate ligands gave [Ir~3~Rh(CO)~8~(μ~2~‐CO)~3~L] (L = Br^–^,2; I^–^, 3; bicyclo[2.2.1]hept‐2‐ene, 4; PPh~3~, 5). Clusters 2–5 have C~s~ symmetry with the ligand L bound to the basal Rh‐atom in axial position. They are fluxional in solution at the NMR time scale due to two CO scrambling processes: the merry‐go‐round of basal CO's and changes of basal face. An additional process takes place in 5 above room temperature: the intramolecular migration of PPh~3~ from the Rh‐ to a basal Ir‐atom. Substitution of CO by polydentate ligands gave [Ir~3~Rh(CO)~7–x~(μ~2~‐CO)~3~(η^4^‐L)x] (L = bicyclo[2.2.1]hepta‐2,5‐diene (= norbornadiene; nbd), x = 1, 6; L = nbd, x = 2, 13; L = cod, x = 1, 7; L = cod x = 2, 15), [Ir~3~Rh(CO)~7~(μ~2~‐CO)~3~(η^2^‐diars)] (diars = 1,2‐phenylenebis‐(dimethylarsine); 8), [Ir~3~Rh(CO)~7~(μ~2~‐CO)~3~(η^4^‐L)] (L = methylenebis(diphenylphosphine), bonded to 2 basal Ir‐atom (9a) or one Ir‐ and one Rh‐atom (9b)), [Ir~3~Rh(CO)~6~(μ~2~‐CO)~3~(η^4^‐nbd)PPh~3~] (12), and [Ir~3~Rh(CO)~6~(μ~2~‐CO)~3~(μ~3~‐L)] (L = 1,3,5‐trithiane, 10; L = CH(PPh~2~)~3~, 11). Complexes 6–8, 9a, 10, and 11 have C~s~ symmetry, the others C~1~ symmetry. They are fluxional in solution due to CO scrambling processes involving 1, 3, or 4 metal centres as deduced from 2D‐EXSY spectra. Comparison of the activation energies of these processes with those of the isostructural Ir~4~ and Ir~2~Rh~2~ compounds showed that substitution of Ir by Rh in the basal face of an Ir~4~ compound slows the processes involving 3 or 4 metal centres (merry‐go‐round and change of basal face), but increases the rate of carbonyl rotation about an Ir‐atom.