A series of homo- and heterotrinuclear complexes containing three face-sharing octahedra has been synthesized by using the pendent arm macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)clononane, L(0)H(3), and 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, LH(3) : [{L(0)Ni(II)}Ni-2(II)](1) and [{LCo(III)}Co-2(III)](PF6)(3) (2); [{LCo(III)}Ni-2](n+) (n = 2 (3), 3 (4), 4 (5)); [{LNi}Co-2(III)](n+) (n = 1 (6), 2 (7), 3 (8)) and its linkage isomers [{LNi}Ni{Co(III)L}](n+) (n = 1 (9), 2 (10), 3 (11)) and, finally, the complexes [{LNi}Ni-2](n+) (n = 0 (12), 1 (13), 2 (14), 3 (15)). In complex 1 three octahedral Ni-II ions form a linear array with two terminal [L(0)Ni(II)](-) moieties in a facial N3O3 donor set and a central Ni-II ion which is connected to the terminal ions via six phenolate bridging pendent arms of L(0). In complexes 2-15 the three metal ions are always in the same ligand matrix yielding an N(3)M(mu-S)(3)M(mu-S)(3)MN(3) first-coordination sphere regardless of the nature of the metal ions (nickel or cobalt) or their formal oxidation states. From temperature dependent magnetic susceptibility measurements it has been determined that 1 has an S = 3 ground state whereas in 12 it is 5 = 1.In order to understand this difference in exchange coupling (ferromagnetic in 1 and antiferromagnetic in 12) in two apparently very similar complexes the magnetic properties of 2-15 have been investigated. Complex 3 has an S = 1 and 4 an S = 1/2, and 5 is diamagnetic (S 0) as is its isoelectronic counterpart 2. This indicates the availability of the oxidation states II, III, and IV of the central NiS6 unit. In the isostructural complexes 6, 7, and 8, two terminal nickel ions are bridged by a central diamagnetic CoIII. The exchange coupling between two terminal paramagnetic nickel ions was studied as a function of their formal oxidation state. In 6 the two Ni-II ions are ferromagnetically coupled (5 = 2); the mixed-valent (NiNiIII)-Ni-II species 7 has an S = 3/2 ground state and in 8 most probably two Ni-III ions (d(7) low spin) give rise to an 5 = 1 ground state. In contrast, in the series 9, 10, and 11 where two nickel ions are in a position adjacent to each other 9 has an S = 0 (antiferromagnetic coupling), but in the mixed-valent complex 10 an S = 3/2 ground state (ferromagnetic coupling) is observed. In fl an S = I ground state prevails which may be achieved by ferromagnetic coupling between two Ni-III ions. For the trinuclear nickel complexes 12-15 an S = 1 ground state has been determined for 12, an S = 3/2 for the mixed valent complex 13, and an S = 2 for 14, and 15 exhibits an S = 3/2 ground state. The Goodenough-Kanamori rules do not provide a consistent explanation for the observed ground states in all cases. The concept of double exchange, originally introduced by Zener in 1951, appears to provide a more appropriate description for the mixed-valent species 7, 10, 13, 14, and 15. This picture is corroborated by the electrochemistry and EPR spectroscopy of complexes.