The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and E)FT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d(8) metal complex, rather than the more common Rh(III) d(6) peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 angstrom range.