The structures and the formation energies of [MCl 4 ] 2- (M = Cr, Mn, Fe, Co, Ni, Zn) have been examined by the density functional method. The optimized structures of [MnCl 4 ] 2- , [CoCl 4 ] 2- and [ZnCl 4 ] 2- are a regular tetrahedron with the M-Cl distances of 236, 227, and 231 pm, respectively. The structures of [CrCl 4 ] 2- and [FeCl 4 ] 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 [NiCl 4 ] 2- is a tetrahedron elongated along an S 1 axis with the Ni-Cl distance of 226 pm and the corresponding angle of 97°. The formation energies of [MCl 4 ] 2- have been compared with their experimental data, and the relative energies of the transformation from [M(H 2 C) 6 ] 2+ to [MCl 4 ] 2- are also examined in connection with the stability of the anions in aqueous solutions.
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