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Transformation among cobalt(II) dichloride hydrates in aqueous solutions was studied by the optical microscopic method. Although there are three hydrates (CoCl2·6H2O, CoCl2·4H2O, and CoCl2·2H2O) in the phase equilibrium diagram, the only solution mediated transformation between CoCl2·6H2O and CoCl2·2H2O could be observed under the present experimental conditions and it was confirmed that CoCl2·4H2O hardly formed compared with the other two hydrates. In order to investigate the reason why CoCl2·4H2O was hardly crystallized at the transformation from CoCl2·6H2O through CoCl2·2H2O and vice versa, the crystal structure of CoCl2·4H2O was investigated by the X-ray diffraction method. The structure of CoCl2·6H2O was reexamined because previously reported results had a relatively large R-factor. The crystal of CoCl2·4H2O is monoclinic, space group P21/a with a=11.548(1) Å, b=9.342(1) Å, c=6.056(1) Å, β=110.79(1)°, and Z=4. The complex has a slightly distorted octahedral geometry about Co2+ ion and two Cl− ions are located at the cis-position. The crystal of CoCl2·6H2O is monoclinic, space group C2/m with a=10.380(2) Å, b=7.048(1) Å, c=6.626(1) Å, β=122.01(1)°, and Z=2. The geometry around the cobalt ion is also octahedral with four water molecules and two chloride ions at the equatorial and the axial positions, respectively. The remaining two water molecules are linked to the two Cl− ions by hydrogen bonding. The difficulty of crystallization of CoCl2·4H2O crystals in solutions was explained in terms of the relatively unstable cis-dichloro structure of the [CoCl2(H2O)4] moiety and its crystal structure stabilized by hydrogen bonding network between the cis-form moieties.

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Full geometry optimizations have been carried out on high-spin hexaaqua complexes of Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ by the local spin density functional method with non-local corrections using Gaussian-type basis sets. The octahedral molecular arrangement around the Mn2+, Ni2+, and Zn2+ ions optimized were regular, whereas those around the Cr2+, Fe2+, Co2+, and Cu2+ ions were distorted with significant differences in the three kinds of M-O distances. The Jahn-Teller distortion for the Fe2+ and Co2+ complexes was firstly found in the field of computational geometry optimization.