Mn(II) complexes of 1,4,7,10,13-pentaazacyclopentadecane ligands are SOD catalysts in which C-substituents on the ligand effect both SOD activity and stability: increasing the number of substituents increases the stability of the complexes while catalytic activity responds unpredictably.Mechanistic studies revealed that these catalysts function via a catalytic cycle in which oxidation of Mn(II) is rate-determining via a H+-coupled electron transfer from a water bound on the Mn(II)(OH2) center to HO2.generating H2O2 and Mn(III)(OH), which is rapidly (diffusion-controlled) reduced to Mn(II) generating O2.This path necessitates that the Mn(II) center have a geometry similar to the oxidized Mn(III) product; thus, accommodating the fast rates we observe for this oxidationMol. mechanics calculations were utilized to probe how ligand substituents influence relative energetics of their Mn(II) complex, altering their geometry.The ability of the ligands to fold about Mn(II) ion adopting a 6-coordinate pseudo-Oh geometry-as required by Mn(III)- correlates with the rate constants for the electron transfer process.