A series of complexes with the formula [Mn-2(III/IV)(mu-O)(2)(L)(2)(X)(2)](3+) have been prepared in situ from (MnLCl2)-L-II precursors by a general preparative method (L = terpy, Cl-terpy, Br-terpy, Ph-terpy, tolyl-terpy, mesityl-terpy, tBu(3)-terpy, EtO-terpy, py-phen, dpya, Me2N-terpy, or HO-terpy, and X = a labile ligand such as water, chloride, or sulfate). The parent complex, where L = terpy and X = water, is a functional model for the oxygen-evolving complex of photosystem 11 (Limburg, et al. J Am. Chem. Soc. 2001, 123, 423-430). Crystals of Mn-II(dpya)Cl-2, Mn-II(Ph-terpy)Cl-2, Mn-II(mesityl-terpy)Cl-2, and an organic-soluble di-mu-oxo di-aqua dimanganese complex, [Mn-2(III/IV)(mu-O)(2)(mesitylterpy)(2)(OH2)(2)](NO3)(3), were obtained and characterized by X-ray crystallography. Solutions of the in situ-formed di-mu-oxo dimanganese complexes were characterized by electrospray mass spectrometry, EPR spectroscopy, and UV-visible spectroscopy, and the rates of catalytic oxygen-evolving activity were assayed. The use of (MnLCl2)-L-II precursors leads to higher product purity of the Mn dimers while achieving the 1:1 ligand to Mn stoichiometry appropriate for catalytic activity assay. These methods can be used to screen the catalytic activity of other di-mu-oxo dimanganese complexes generated by using a ligand library.