Bonding and Magnetic Exchange Pathways in Nature's Water-Oxidizing Complex

The nature of the bonding and magnetic exchange pathways of the water-oxidizing complex of photosystem 2 is explored using broken symmetry density functional theory. The electronic structure and superexchange pathways are illustrated and analyzed using corresponding orbitals and intrinsic bond orbitals. These demonstrate a dominating influence on the bonding and magnetic interactions by both the geometrical structure of the Mn₄CaO₅ core complex and the ionic interactions of the oxo bridges with the neighboring Ca²⁺ ion. The demonstrated ionic nature of the Ca²⁺ bonds is proposed to contribute to the stabilization of the oxygen atoms participating in O-O bond formation.