OXO-CENTERED TRINUCLEAR ACETATE COMPLEXES CONTAINING MIXED-METAL CLUSTERS - CRYSTAL-STRUCTURE OF A CHROMIUM(III)IRON(III)NICKEL(II) COMPLEXAND MAGNETIC-PROPERTIES OF A DICHROMIUM(III)MAGNESIUM(II) COMPLEX

The crystal structure of rxFe2-xNiO)-Fe-III-Ni-III-O-II(O2CCH3)(6)(py) (3)]. py 1 (x = ca. 0.7, py = pyridine), has been determined at 25 deg rees C. Although the molecule has no crystallographically imposed symm etry the metal atoms form an almost equilateral triangle, M ... M 3.27 4(3), 3.284(4), 3.281(3) Angstrom, with the O atom less than 0.006 Ang strom from the M-3 plane, and the Cr. Fe, and Ni atoms extensively dis ordered over the three metal sites. The corresponding Cr2Mg, Cr2Ni, an d Fe2Ni complexes are isomorphous with 1. The structure differs from t hat of the rhombohedral (Fe2MII)-M-III (M-II = Mn, Fe, Co or Zn) and ( Cr2MII)-M-III (M-II = Mn, Fe or Co) analogues in the orientations of t he pyridine rings. The planes of the pyridine ligands in 1 are approxi mately perpendicular to the M-3 plane, and the non-coordinated pyridin e molecule lies approximately in this plane. (This contrasts with the rhombohedral complexes, where the ligand pyridine rings are approximat ely parallel with the M-3 plane and the non-co-ordinated pyridine lies with its plane on the three-fold axis of the M3O system.) The magneti c susceptibility of [(Cr2MgO)-Mg-III(O2CCH3)(6)(py)(3)] py has been me asured between 5 and 300 K, and fitted by use of an exchange hamiltoni an -2JS(1) . S-2, giving J(CrCr) = -17 cm(-1), a value considerably sm aller than that (ca. -26 cm(-1)) of the Cr2Ni and Cr2Co analogues.