Preparation and physical properties of trinuclear oxo-centered manganese complexes of general formulation Mn3O(O2CR)6L30,+ (R = methyl or phenyl; L = a neutral donor group) and the crystal structures of Mn3O(O2CMe)6(pyr)3(pyr) and Mn3O(O2CPh)6(pyr)2(H2O).cntdot.0.5MeCN