The geometric and electronic structures of two mononuclear [(L)CoO 2]+ complexes, [(12-TMC)CoO2](ClO4) (1) and [(14-TMC)CoO2](ClO4) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are CoIII species. Co K-edge EXAFS data show that both 1 and 2 are side-on O2-bound cobalt(III) peroxide complexes. A combination of EXAFS and DFT calculations reveals that while the constrained 12-TMC ring in 1 allows for side-on O2 binding to the Co center with ease, the 14-TMC chelate in 2 has to undergo significant distortion of the ring to overcome steric hindrance posed by the four cis-methyl groups of the chelate to allow side-on O2 binding to the Co center. The Ni analogue of 2, [(14-TMC)NiO2]+, has been shown to form an end-on-bound nickel(II) superoxide species. The electronic and geometric factors that determine the different electronic structures of 2 and [(14-TMC)NiO 2]+ are evaluated using DFT calculations. The results show that while the sterics of the cis-14-TMC chelate contribute to the geometry of O2 binding and result in an end-on-bound NiIIO 2- complex in [(14-TMC)NiO2]+, the higher thermodynamic driving force for oxidation of CoII overcomes this steric constraint, resulting in stabilization of a side-on-bound Co IIIO22- electronic structure in 2.