ABSTRACT Full geometry optimizations for some octahedral cobalt (II) chloride complexes, [Co(H2O)6]2+, [CoCl(H2O)5]+, cis- and trans-[CoCl2(H2O)4], and fac- and mer-[CoCl3(H2O3)]-, cis and trans-[CoCl4(H2O)2]2-, and tetrahedral ones, [CoCl2(H2O)2], [CoCl3(H2O)]- and [CoCl4]2-, were carried out by the self-consistent nonlocal spin density functional method with Becke-Lee-Yang-Perdew nonlocal corrections. The geometries optimized for most of the octahedral and tetrahedral complexes were retained with in the respective starting geometries with slight distortions, although that of cis-[CoCl4(H2O)2]2- was completely broken and was led to the tetrahedral tetrachloro complex of [CoCl4]2- with removal of two water molecules from Co2+ ion. Among of the optimized complexes, the most stable octahedral and tetrahedral species were [CoCl3(H2O)3]- and [CoCl3(H2O)]-, respectively. On the basis of consideration of the intramolecular hydrogen bond energies, the process of conversion from octahedral to tetrahedral complexes in aqueous solution has been also discussed.
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