Manganese ions are essential for the photosynthetic conversion of water to dioxygen via the enzyme known as the oxygen evolving complex. While the structure of the active center has not yet been resolved, it is believed that the four manganese ions are in close proximity for most if not all of the oxidation levels (S n states) of the enzyme. One structural model that has gained some popularity is the dimer of dimers organization proposed by the research groups of Mel Klein and Ken Sauer. While still unproven, this structural proposal forms the basis to begin studies of manganese reactivity. Two limiting mechanisms that have been proposed for water oxidation are attack of water (hydroxide) onto a terminal oxo of a manganyl unit (Mn=O) or the collapse of two oxo bridges to form peroxide which could then react with a second dimer to form dioxygen.In this presentation we will focus on the chemistry of Mn dimers in the Mn(III) 2 , Mn(III)Mn(IV), and Mn(IV) 2 oxidation levels that contain imidazolate bridges (see below). We are interested in these compounds as it has been suggested, based on pulsed EPR experiments, that a histidine imidazole may bridge two of the manganese ions in this system. We have prepared six complexes that are related over three oxidation levels and two protonation states. We will examine the structures and physical properties of these molecules and compare them to related di-μ 2 -oxo bridged dimers.