Formation of [CoCl4(H2O)2]2- complex in CoCl2•MgCl2•8H2O double salt:: structural and energetic properties of [MCl4(H2O)2]2- and [M(H2O)6]2+ (M = Mg, Co)

The crystal structure of the double salt COCl2 . MgCl2 . 8H(2)O has been determined by the X-ray diffraction method. It crystallizes in the space group P (I) over bar with a = 6.0976(9), b = 6.308(1), c = 8.579(3) Angstrom, 81.99(2)degrees, 88.40degrees, gamma = 84.61 (1)degrees, Z = 1, and R = 0.027. The crystal consists of two kinds of well separated octahedra, [COCl4(H2O)(2)](2-) and [Mg(H2O)(6)](2+). The former is unique as aquachloro complexes of Co2+. In order to elucidate the reason prepared as such unique complexes in the double salts, formation energies for [MCl4(H2O)(2)](2-) and [M(H2O)(6)](2+) (M=Co, Mg) have been calculated by using the density functional methods, and it has been revealed that the formation energies of the first coordination sphere for the metal ions and the Cl- ... H2O hydrogen bond networks around [CoCl4(H2O)(2)](2-) play a decisive role in forming [CoCl4(H2O)(2)](2-) with the regular octahedral geometry in the double salt. (C) 2003 Elsevier Science (USA). All rights reserved.