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.