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)-1,4,7-triazacyclononane, L0H3, and 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, LH3:  [{L0NiII}2NiII] (1) and [{LCoIII}2CoIII](PF6)3 (2); [{LCoIII}2Ni] n + (n = 2 (3), 3 (4), 4 (5)); [{LNi}2CoIII] n + (n = 1 (6), 2 (7), 3 (8)) and its linkage isomers [{LNi}Ni{CoIIIL}] n + (n = 1 (9), 2 (10), 3 (11)) and, finally, the complexes [{LNi}2Ni] n + (n = 0 (12), 1 (13), 2 (14), 3 (15)). In complex 1 three octahedral NiII ions form a linear array with two terminal [L0NiII]- moieties in a facial N3O3 donor set and a central NiII ion which is connected to the terminal ions via six phenolate bridging pendent arms of L0. In complexes 2−15 the three metal ions are always in the same ligand matrix yielding an N3M(μ-S)3M(μ-S)3MN3 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 S = 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 NiII ions are ferromagnetically coupled (S = 2); the mixed-valent NiIINiIII species 7 has an S = 3/2 ground state and in 8 most probably two NiIII ions (d7 low spin) give rise to an S = 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 11 an S = 1 ground state prevails which may be achieved by ferromagnetic coupling between two NiIII 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.

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A series of heterodinuclear complexes CunM, where M = Cr(III) (1), Mn(III) (2), Mn(II) (3), Fe(III) (4), Co(II) (5), Co(IlI) (6), Ni(II) (7), Cu(II) (8), and Zn(II) (9), containing the oximato dianion (Dopn* 12~) as bridging ligand and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as one of the two capping ligands have been synthesized by using the [Cu(DopnH)]+ cation (H2Dopn = 3,9-dimethyl-4,8-diazaundeca-3,8-diene-2,10-dione dioxime) as a ligand for the different ML"+ centers. The compounds have been characterized on the basis of IR, electronic, and EPR spectroscopy and variable-temperature (2-295 K) magnetic susceptibility measurements. The dinuclear complexes are quasi-isostructural with the copper(H) ion in a distorted square pyramidal environment, CuN40, and the M ion, except for that in 8, is six-coordinate with the MN3O3 or MN3O2CI coordination sphere. For M = Cu (8), the coordination geometry of M with the CuN302 chromophore is also square pyramidal. The crystal and molecular structures of the compounds [(Dopn)CuI1(OH2)CrIII(OCH3)L](C104)2,H20 (1) and [(Dopn)Cuu(u-CH3COO)-MnnIL] (C104)r2H20 (2) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system space group P2i/n, with cell constants a = 13.096(3) A, b= 17.933(4) A, c= 15.994(3) A, /3= 113.49(3)°, V= 3444.9(13) A3 4, and Z = 4. The structure consists of oximato-bridged CunCrIn dications and noncoordinated perchlorate anions, with a Cu-• -Cr distance of 3.86 A. The crystal data for 2 are as follows: orthorhombic, space group Z>212121, a = 12.275(4) A,b = 14.171(9) A, c = 19.780(3) A, V -3441(2) A3, Z = 4. The structure consists of a six-coordinate Mn(III) center, MnN3C>3, and the copper(II) center has an N40 donor set. An acetate group bridges the manganese and copper ions with a Cu-• -Mn separation of 3.54 A. A low-quality X-ray structure determination for the CunFein complex is also reported. Analysis of the susceptibility data yields a strong antiferromagnetic interaction (27 = -596 cm-1) between adjacent Cu(II) ions in 8, showing once again that bridging oximes are good mediators for exchange interactions. The strength of the effective antiferromagnetic interaction decreases with increasing number of unpaired electrons in this series, 8>7>5>4~3.Moderately strong ferromagnetic interactions have been observed for CunMnIU (2) (27 = +109 cm-1) and CunCrin (1) (27 = +37 cm-1). A qualitative rationale has been provided for the difference in magnetic behaviors. The X-band EPR spectra (3-77 K) have been measured to establish the ground states of the dinuclear complexes. Well-resolved S = 2 EPR spectra for different heterometal systems have been observed.

The reaction of Cu(MeC02)2.H20 with 1,4,7-trimethyl-I ,4,7-triazacyclononane (L) and imidazole (ImH) in methanol affords, in the presence of a small amount of triethylamine and NaC104, the blue trinuclear complex [L3C~3(lm)3](C104)3 which has been characterized by X-ray crystallography, EPR and magnetic susceptibility measurements to be a spin-frustrated triangular imidazolate-bridged copper(ii) complex without any p3-X ligand.

In dinuclear transition-metal complexes the connection of the coordination polyhedra of two metal ions with octahedral geometry by face-sharing is a commonly observed structural feature. Metal. . . metal distances can range from 2.3 to 4.0 A depending on the type of the three bridging ligands, which are often composed of one atom or coordinate only with one atom, for example 0'-, OH-, RO-, C1-, Br-, and I- (Table 1). If both metal ions possess unpaired electrons then intramolecular spin-exchange interactions and ferromagnetic or antiferromagnetic coupling results. This exchange interaction can be satisfactorily described for metals of the first transition-series by the isotropic HeisenbergDirdc-van Vleck model with the spin-Hamiltonion operator H = -2 JS, . S,; here S, and S, are the total spins on ions 1 and 2, and J is the spin-exchange coupling constant in cm-'.['] The value of J can be obtained from the temperature-dependence of the magnetic susceptibility of a complex. Antiferromagnetic coupling arises for homodinuclear complexes in which the two metals have d3-d3 electronic configurations; at low temperatures these complexes are diamagnetic (S = 0, ground state). The energy difference between the singlet state (S = 0) and the first excited triplet-state (S = 1) is expressed by 2J.[ Although many face-sharing d3_d3 dinuclear complexes have been structurally characterized, and temperaturedependent susceptibility measurements have been carried out, no correlation has been found to date between structural parameters and the magnitude of J. 1lc1 We demonstrate here that the only relevant structural parameter that correlates with the value of J i s the distance between the two metal ions.We obtained 2 by protonation of an 0x0 bridge in 1['] (L = N,N',N''-trimethyl-l,4,7-triazacyclononane) with conc.