Mn-4/(AE)(2) complexes, AE=Ca,1 or Ba,2, display (Mn3MnII)-Mn-III valence-trapped states and Jahn-Teller compressed Mn-III ions, consistent with the S-0 (or S-2) oxidation level of PSII. Ba2+ and Ca2+ ligate Mn-bonded carboxylate oxygens either directly or via H2O molecules, respectively. The (O-H-O)(3-) ligand, symmetric, mu(4), as in 1, is asymmetric, mu(5), in 2, part of a mu(3)-oxo Mn2Ba unit. AE dependent acetate ligation to Mn is also observed. 1 and 2 have biomimetic catalase activity.