An unexpected cluster opening upon the formation of electronically unsaturated η3-(cyclooctenyl)metallacarboranes of rhodium(III) and iridium(III) with sterically reduced [(PhCH2)2C2B9H9]2− ligand

The room-temperature metallation reactions of the K+ salt of the [7,8-(PhCH2)2-7,8-nido-C2B9H10]− anion (1) with the COD-metal μ-chloride dimers [(η4-C8H12)2Rh2(μ-Cl)2] (2) and [(η4-C8H12)2Ir2(μ-Cl)2] (3) in benzene/ethanol solution gave formally 16-electron pseudocloso-type complexes with the η3-cyclooctenyl ligand at the metal vertices, [3-{(1–3-η3)-C8H13}-1,2-(PhCH2)2-pseudocloso-3,1,2-MC2B9H9] [4, M = Rh(III); 5, M = Ir(III)]. No evidence supporting the existence of an agostic C–H⋯M bonding interaction in these compounds was obtained either from the crystallographic or the phase-sensitive 2-D [1H–1H] NOESY/EXSY studies of 4. The extraordinary stability of complexes 4 and 5 can therefore be associated with their cage-deformed cluster structures, where electronically-deficient (16-electron) metal centers are believed to be stabilized by additional electron density released from the polyhedral C–C bond cleavage. DFT solid-state calculations performed for closo (18-electron) and pseudocloso (16-electron) Rh(III) complexes, [3-(η5-C5Me5)-1,2-(PhCH2)2-closo-3,1,2-RhC2B9H9] (6, C–C, 1.7397 Å) and [3-{(1–3-η3)-C8H13}-1,2-(4′-MeC6H4)2-pseudocloso-3,1,2-RhC2B9H9] (9, C⋯C, 2.420(2) Å), showed that the electron density transfer from the carborane moiety to the rhodium center is marginally greater for complex 9, in accordance with the idea that electronics rather than sterics play a crucial role in the stabilization of 16-electron pseudocloso-metallacarborane species.