Rate enhancements for the reduction of dioxygen by a Mn-II complex were observed in the presence of redox-inactive group 2 metal ions. The rate changes were correlated with an increase in the Lewis acidity of the group 2 metal ions. These studies led to the isolation of heterobimetallic complexes containing Mn-III-(mu-OH)-M-II cores (M-II = Ca-II, Ba-II) in which the hydroxo oxygen atom is derived from O-2. This type of core structure has relevance to the oxygen-evolving complex within photosystem II.