Structure and Bonding of a Zwitterionic Iridium Complex Supported by a Phosphine with the Parent Carba-closo-dodecaborate CB₁₁H₁₁^– Ligand Substituent

A zwitterionic iridium complex of a phosphine, bearing the carba-closo-dodecaborate anion as a ligand substituent, is reported. When tethered directly to a phosphine ligand, the CB₁₁H₁₁^– R group engages in agostic-like bonding, utilizing the B–H bonds adjacent to the carbon atom in the cluster. Evidence for the interactions is observed in solution by variable-temperature NMR and also in the solid state by a single-crystal X-ray diffraction study. The bonding between the cluster and the iridium center has also been analyzed computationally and can be described as a double agostic bonding augmented by an overlap of the skeletal electrons of the cluster with the d_z² orbital of the metal. A significant lengthening of a trans-olefin C–C bond suggests that this ligand substituent has a pronounced trans influence, which is in contrast to the unfunctionalized weakly coordinating HCB₁₁H₁₁^– anion.