EXCHANGE AND DOUBLE-EXCHANGE PHENOMENA IN LINEAR HOMOTRINUCLEAR AND HETEROTRINUCLEAR NICKEL(II,III,IV) COMPLEXES CONTAINING 6 MU(2)-PHENOLATO OR MU(2)-THIOPHENOLATO BRIDGING LIGANDS

A series of homo- and heterotrinuclear complexes containing three face -sharing octahedra has been synthesized by using the pendent arm macro cyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)clononane, L(0) H(3), and t-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(I II)}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-I I 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 m atrix yielding an N(3)M(mu-S)(3)M(mu-S)(3)MN(3) first-coordination sph ere regardless of the nature of the metal ions (nickel or cobalt) or t heir formal oxidation states. From temperature dependent magnetic susc eptibility measurements it has been determined that 1 has an S = 3 gro und state whereas in 12 it is 5 = 1. In order to understand this diffe rence in exchange coupling (ferromagnetic in 1 and antiferromagnetic i n 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 I V 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 w as studied as a function of their formal oxidation state. In 6 the two Ni-II ions are ferromagnetically coupled (5 = 2); the mixed-valent (N iNiIII)-Ni-II species 7 has an S = 3/2 ground state and in 8 most prob ably two Ni-III ions (d(7) low spin) give rise to an 5 = 1 ground stat e. 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 pre vails which may be achieved by ferromagnetic coupling between two Ni-I II ions. For the trinuclear nickel complexes 12-15 an S = 1 ground sta te 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 exchan ge, originally introduced by Zener in 1951, appears to provide a more appropriate description for the mixed-valent species 7, 10, 13, 14, an d 15. This picture is corroborated by the electrochemistry and EPR spe ctroscopy of complexes.