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‡

Abstract

The crystal structure of [Cr^III_xFe^III_2–xNi^IIO(O₂CCH₃)₆(py)₃]·py 1 (x = ca. 0.7, py = pyridine), has been determined at 25 °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) Å, with the O atom less than 0.006 Å from the M₃ plane, and the Cr, Fe, and Ni atoms extensively disordered over the three metal sites. The corresponding Cr₂Mg, Cr₂Ni, and Fe₂Ni complexes are isomorphous with 1. The structure differs from that of the rhombohedral Fe^III₂M^II (M^II = Mn, Fe, Co or Zn) and Cr^III₂M^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 M₃ plane, and the non-co-ordinated pyridine molecule lies approximately in this plane. (This contrasts with the rhombohedral complexes, where the ligand pyridine rings are approximately parallel with the M₃ plane and the non-co-ordinated pyridine lies with its plane on the three-fold axis of the M₃O system.) The magnetic susceptibility of [Cr^III₂MgO(O₂CCH₃)₆(py)₃]·py has been measured between 5 and 300 K, and fitted by use of an exchange hamiltonian –2JS₁·S₂, giving J_CrCr = –17 cm^–1, a value considerably smaller than that (ca. –26 cm^–1) of the Cr₂Ni and Cr₂Co analogues.