Application of density functional calculations to the structures and formation energies of [MCl₄]^2- complexes (M=Cr, Mn, Fe, Co, Ni, Zn)

The structures and the formation energies of [MCl₄]^2- (M = Cr, Mn, Fe, Co, Ni, Zn) have been examined by the density functional method. The optimized structures of [MnCl₄]^2-, [CoCl₄]^2- and [ZnCl₄]^2- are a regular tetrahedron with the M-Cl distances of 236, 227, and 231 pm, respectively. The structures of [CrCl₄]^2- and [FeCl₄]^2- are a flattened tetrahedron with the M-Cl distances of 236 and 231 pm, respectively, and with the flattening angles of 156 and 112°, respectively, while that of [NiCl₄]^2- is a tetrahedron elongated along an S₁ axis with the Ni-Cl distance of 226 pm and the corresponding angle of 97°. The formation energies of [MCl₄]^2- have been compared with their experimental data, and the relative energies of the transformation from [M(H₂C)₆]²⁺ to [MCl₄]^2- are also examined in connection with the stability of the anions in aqueous solutions.