Synthesis and Characterization of Cationic Rhodium Peroxo Complexes

Mononuclear cationic rhodium complexes of dioxygen have been synthesized and characterized. Crystallographic, spectroscopic, and computational results support the conclusion that these complexes are best described as RhIII{O22–} (rhodium­(III) peroxo) complexes, in contrast to recently reported neutral analogues that are best described as RhI{1O2} adducts. The nature of the ligand trans to the O2 ligand is crucial in defining the electronic nature of the RhO2 bonding. It is determined that π-donor ligands such as the halidesin conjunction with sufficient steric bulkcan stabilize the formation of RhI{1O2} adducts, whereas stronger field ligands lead to the stabilization of asymmetric O2 binding that ultimately favors formation of higher coordinate RhIII peroxo species. The factors that control the relative stabilization of RhIII{O22–} versus RhI{1O2} species are related to the well-established Dewar–Chatt–Duncanson model that has been successfully used to describe the bonding in isoelectronic transition-metal alkene complexes. The specific factors that control the stabilization of one electromer (resonance structure) over another are explored and discussed in detail.