Preparation and physical properties of trinuclear oxo-centered manganese complexes of general formulation [Mn3O(O2CR)6L3]0,+ (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