The most stable structures and formation energies of [M(NCS)(4)](2-), [M(NCS)(2)(SCN)(2)](2-), [M(SCN)(4)](2-) (M = Zn(II), Cd(II), Hg(II)) and [Cd(NCS)(3)(SCN)](2-) have been calculated by the ab initio density functional method. The complex anions [M(NCS)(4)](2)- were optimized to regular tetrahedral geometry with a linear M-N-C, and the [M(SCN)(4)](2-) complex anions to a twisted tetrahedral geometry. The geometry optimizations of [M(NCS)(2)(SCN)(2)](2-) and [Cd(NCS)(3)(SCN)](2-) contained both linear M-N-C and bent M-S-C bonds formed in the optimized complexes The formation energies of [M(NCS)(4)](2-), [M(NCS)(2)(SCN)(2)](2-) and [M(SCN)(4)](2-) are - 2701, - 2625 and - 2557 kJ mol(-1) for Zn(II), - 2378, - 2317 and - 2262 kJ mol(-1) for Cd(II), and - 3803, - 4082 and - 4333 kJ mol(-1) for Hg(II), respectively, and that of [Cd(NCS)(3)(SCN)](2-) is - 2342 kJ mol(-1). A comparison of the formation energies indicated that both in water and in dimethyl sulfoxide [Zn(NCS)(3)](2-) and [Hg(SCN)(4)](2-) are the most stable complexes among the respective coordination isomers. However, the complex anions [Cd(NCS)(3)(SCN)](2-) and [Cd(NCS)(2)(SCN)(2)](2-) in dimethylformamide and in water, respectively, were less stable than [Cd(NCS)(4)](2-). The tetrathiocynato metal complexes with several coordination modes of SCN in solution were discussed on the basis of their formation energies.
