Towards models of the oxygen-evolving complex (OEC) of photosystem II: a Mn4Ca cluster of relevance to low oxidation states of the OEC.

Synthetic access has been achieved into high oxidation state Mn/Ca chemistry with the 4 : 1 Mn : Ca stoichiometry of the oxygen-evolving complex (OEC) of plants and cyanobacteria; the anion of (Et(3)NH)(2)[Mn(III)(4)Ca(O(2)CPh)(4)(shi)(4)] has a square pyramidal metal topology and an S = 0 ground state.