INTRAMOLECULAR DYNAMICS OF TETRANUCLEAR IRIDIUM CARBONYL CLUSTER COMPOUNDS .5. POLYSUBSTITUTED DERIVATIVES OF [IR-4(CO)(12)]

The dynamic behaviour of twelve polysubstituted derivatives of [Ir-4(C O)(12)] has been investigated in solution, using 2D-EXSY, and VT-P-31- and C-13-NMR. [Ir-4(CO)(6)(mu(2)-CO)(3)(eta(4)-diarsine)PPh(3)] and ) (6)(mu(2)-CO)(3)(eta(4)-norbornadiene)(PMePh(2))] exhibit two isomeric forms in solution, which interconvert through an intramolecular chang e of basal face. The related cluster (CO)(6)(mu(2)-CO))(3)(eta(4)-norb ornadiene)PPh(3)] exists as a single isomer in solution. It displays r otation of CO ligands about the apical Ir-atom, followed by two consec utive changes of basal face. The tetrasubstituted clusters with two ch elating ligands [Ir-4(CO)(5)(mu(2)-CO)(3)(eta(4)-diolefin)(2)] also ex hibit relation of apical CO's, the activation energy increases with gr eater steric hindrance of the radial ligands. A quantitative analysis of the P-31- and C-13-2D-EXSY spectra followed by simulation of the co rresponding VT-NMR spectra of [Ir-4(CO)(5)(mu(2)-CO)(3)(mu(2)-L)(2)] ( L = bis(diphenylphosphino)methane and 1,3-bis(diphenylphosphino)propan e) revealed a pairwise averaging of the P-atoms, caused by two paralle l changes of basal face averaging all CO ligands. In addition, the res tricted rotation of ligands about the apical Ir-atom was identified at higher temperatures. The remaining clusters are either rigid on the N MR time scale, or display CO-scrambling about a single Ir-atom.