Oxo-centred trinuclear acetate complexes containing mixed-metal clusters. Crystal structure of a chromium(III)iron(III)nickel(II) complex and magnetic properties of a dichromium(III)magnesium(II) complex

The crystal structure of [Cr(x)(III)Fe(2-x)(III)Ni(II)O(O(2)CCH(3))(6)(py)(3)]. py 1 (x = ca. 0.7, py = pyridine), has been determined at 25 degrees C. Although the molecule has no crystallographically imposed symmetry the metal atoms form an almost equilateral triangle, M ... M 3.274(3), 3.284(4), 3.281(3) Angstrom, with the O atom less than 0.006 Angstrom from the M(3) plane, and the Cr. Fe, and Ni atoms extensively disordered over the three metal sites. The corresponding Cr(2)Mg, Cr(2)Ni, and Fe(2)Ni complexes are isomorphous with 1. The structure differs from that of the rhombohedral Fe(2)(III)M(II) (M(II) = Mn, Fe, Co or Zn) and Cr(2)(III)M(II) (M(II) = Mn, Fe or Co) analogues in the orientations of the pyridine rings. The planes of the pyridine ligands in 1 are approximately perpendicular to the M(3) plane, and the non-coordinated pyridine molecule lies approximately in this plane. (This contrasts with the rhombohedral complexes, where the ligand pyridine rings are approximately parallel with the M(3) plane and the non-co-ordinated pyridine lies with its plane on the three-fold axis of the M(3)O system.) The magnetic susceptibility of [Cr(2)(III)MgO(O(2)CCH(3))(6)(py)(3)] py has been measured between 5 and 300 K, and fitted by use of an exchange hamiltonian -2JS(1) . S(2), giving J(CrCr) = -17 cm(-1), a value considerably smaller than that (ca. -26 cm(-1)) of the Cr(2)Ni and Cr(2)Co analogues.