The reaction of the bis-chelating ligand 1,2-bis(2,2'-bipyridine-6-yl)ethane (L) with the trinuclear species of formula [Mn3O(O2CR)(6)(PY)(3)](ClO4) (R = Me (1); R = Et (2); R = Ph (3)) has afforded the new tetranuclear mixed-valent complexes [Mn4O2(O2CR)(4)L-2](ClO4)(2) (R = Me (4); R = Et (5); R = Ph (6)) and [Mn4O2(OMe)(3)(O2CR)(2)L-2(MeOH)]-(ClO4)(2) (R = Me (7); R = Et (8); R = Ph (9)). Complexes 4-6 were obtained in yields of 20%, 44%, and 37%, respectively. They are mixed-valent, with an average Mn oxidation state of +2.5, Complexes 7-9 were obtained in yields of 57%, 65%, and 70%, respectively, They are also mixed-valent, but with an average Mn oxidation state of +2.75. Complexes w4.2THF and 9.3MeOH.H2O Crystallize in the triclinic space group P (1) over bar and contain [Mn-4(mu(3)-O)(2)](6+) and [Mn-4(mu(3)-O)(2)(mu-OMe)(2)](5+) cores, respectively, the latter being a new structural type in the family of Mn4 complexes. Reactivity studies of 4-9 have shown that 4-6 can be converted into 7-9, respectively, and vice versa, The magnetic properties of 5 and 9 have been studied by do and ac magnetic susceptibility techniques. Complex 5 displays antiferromagnetic coupling between its Mn ions resulting in a spin ground state of S = 0. Complex 9 also displays antiferromagnetic coupling, but the resulting ground state is S 7/2, as confirmed by fitting magnetization versus field data collected for 9 at low temperatures, which gave S 7/2, D = -0.77 cm(-1), and g = 1.79. Complex 9 exhibits a frequency-dependent out-of-phase ac susceptibility peak, indicative of the slow magnetization relaxation that is diagnostic of single-molecule magnetism behavior.