Crystal and Molecular Structures of Trans-tetrakis(pyridine)dichloroiron(II), -nickel(II), and -cobalt(II) and Trans-tetrakis(pyridine)dichloroiron(II) Monohydrate


The crystal and molecular structures of Fe(py)4Cl2, Co(py)4Cl2, Ni(py)4Cl2, and Fe(py)4Cl2·H2O have been determined from three-dimensional x-ray diffraction data collected on a four-circle diffractometer. The first three compounds crystallize in the tetragonal I41/acd space group with unit cell parameters of ca. 15.9 Å for a and ca. 17.1 Å for b and a volume of ca. 4350 Å3. Fe(py)4Cl2·H2O crystallizes in the monoclinic P21/c space group with a = 9.384 (6) Å, b = 16.766 (4) Å, c = 16.342 (9) Å, ß = 121.497 (25)°, and V = 2192.2 (1.2) Å.3 The structures were solved by Patterson and Fourier methods and refined where feasible by full-matrix least-squares procedures. All nonhydrogen atoms were refined with anisotropic thermal parameters and all pyridine hydrogen atoms were located by electron difference methods although more accurate positions were derived through geometric considerations; the resulting conventional R factors are 4.77, 4.83, 4.98, and 3.88%, respectively. In each complex the metal is coordinated in a trans fashion to two chloride ions and four pyridine molecules. The molecules, which possess 222 but not 4 symmetry, are oriented with their pseudotetragonal (Cl-M-Cl) axis normal to the tetragonal axis of the crystals. The individual molecules of M(py)4Cl2 are well isolated from each other with essentially no intermolecular contact distances of less than the sum of van der Waals radii. The one exception to this is Fe(py)4Cl2·H2O where the water molecule serves as a presumably hydrogen-bonded bridge between two chloride ions in adjacent molecules. In the anhydrous iron, cobalt, and nickel complexes the M-Cl bond distances are 2.430 (3), 2.440 (2), and 2.437 (3) Å, and the M-N bond distances are 2.229 (6), 2.183 (4), and 2.133 (4) Å, respectively. The decrease in the M-N bond distance with increasing metal atomic number is attributed to the importance of the increasing number of t2g π-bonding electrons in the metal ions. The relatively constant M-Cl bond distance is attributed to the constant number of σ-bonding electrons in these metal ions. The dihedral angle between the plane of the coordinate nitrogen atoms and the pyridine ring is ca. 51° in the anhydrous complexes. The increase of this dihedral angle above 45° is attributed either to a pyridine hydrogen to chlorine hydrogen-bonded interaction or to a hydrogen-carbon interaction on adjacent coordinated pyridine molecules.



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