The complexes {[bispicen)Mn]2(mu-O)(mu-O)(mu-OAc)}-(ClO4)2 (1), where bispicen is N,N’-bis(2-pyridylmethyl)ethane-1,2-diamine, and {[bispicMe2en)-Mn]2(mu-O)(mu-OAc)}(ClO4)3 (2), where bispicMe2en is N,N’-bis(2-pyridylmethyl)-N,N’-dimethylethane-1,2-diamine, are described. Both complexes contain the biologically-significant (mu-oxo)(mu-acetato)dimanganese(III/III) core, the coordination at manganese being completed by the ligation of four nitrogen atoms from the tetradentate ligands. The Mn-Mn separations are 3.276(3) and 3.29(1) angstrom in 1 and 2, respectively, close to the value of 3.3 angstrom estimated from EXAFS data for one of the Mn...Mn separations in PSII OEC.