X-ray structural studies of FeBr 2·4H 2O, CoBr 2·4H 2O, NiCl 2·4H 2O and CuBr 2·4H 2O. cis/trans selectivity in transition metal(II) dihalide tetrahydrate

Abstract

Crystal structures of FeBr 2·4H 2O ( 1), CoBr 2·4H 2O ( 2), NiCl 2·4H 2O ( 3) and CuBr 2·4H 2O ( 4) have been determined by the X-ray diffraction method. 1 crystallizes in the monoclinic form with the space group P2 1/ a having the unit cell dimensions of a=8.730(2)x10 2, b=7.239(1)x10 2, c=6.149(2)x10 2 pm, β=110.72(2)°, V=363.5x10 6 pm 3 and Z=2; 2 monoclinic, P2 1 / a, a=8.581(5)x10 2, b=7.143(4)x10 2, c=6.180(2)x10 2 pm, β=110.29(4)°, V=355.3x10 6 pm 3 and Z=2; 3 monoclinic, P2 1/ a, a=13.378(3)x10 2, b=9.301(3)x10 2, c=5.988(1)x10 2 pm, β=126.45(1)°, V=599.4x10 6 pm 3 and Z=4; 4 monoclinic, P2 1/ a, a=12.013(3)x10 2, b=7.330(4)x10 2, c=4.116(2)x10 2 pm, β=105.44(3)°, V=349.3x10 6 pm 3 and Z=2. The hexa-coordinated geometry around the Fe 2+ and Co 2+ ions in 1 and 2, respectively, is a distorted trans-octahedron with four aqua oxygens at the equatorial position and two Br − ions at the axial position, whereas the hexa-coordinated central Ni 2+ ion in 3 has a distorted cis-octahedral geometry with three aqua oxygens and one Cl − ion at the equatorial position and one aqua oxygen and one Cl − ion at the axial position. The central Cu 2+ ion in 4 has an axially elongated octahedral form with two Br − ions and two aqua oxygens at the equatorial position, and two Br − ions belonging to the adjacent units locate at the axial position of the copper(II) ion. The remaining two water molecules in the compound are situated outside the coordination sphere of the copper(II) ion and are connected to the water molecule in the coordination sphere of a copper(II) ion with a Br − ion in an adjacent copper(II) complex by hydrogen bonds. The cis/trans selectivity in the transition metal(II) dihalide tetrahydrate complexes is discussed in terms of the electronic configuration of the central metal(II) ions, the repulsive interaction between halide ions due to the steric effect, and the effect of hydrogen bonds.