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 Rh ^III{O ₂ ^2-} (rhodium(III) peroxo) complexes, in contrast to recently reported neutral analogues that are best described as Rh ^I{ ¹O ₂} adducts. The nature of the ligand trans to the O ₂ ligand is crucial in defining the electronic nature of the RhO ₂ bonding. It is determined that π-donor ligands such as the halides-in conjunction with sufficient steric bulk-can stabilize the formation of Rh ^I{ ¹O ₂} adducts, whereas stronger field ligands lead to the stabilization of asymmetric O ₂ binding that ultimately favors formation of higher coordinate Rh ^III peroxo species. The factors that control the relative stabilization of Rh ^III{O ₂ ^2-} versus Rh ^I{ ¹O ₂} 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.