X-Ray Diffraction, Raman, and NMR Studies on Tetrathiocyanato Complexes of Zinc(II), Cadmium(II), and Mercury(II) Ions in Aqueous Solution

Abstract

Abstract X-Ray scattering, Raman, and 13C- and 14N-NMR measurements were performed on aqueous solutions containing tetrathiocyanato complex ions of Zn(II), Cd(II), and Hg(II). The radial distribution functions revealed that four thiocyanate ions tetrahedrally coordinated to Zn(II) and to Hg(II) ions through the N and S atoms, respectively, while Cd(II) ion was bound to two sulfur and two nitrogen atoms within the coordinated four SCN− ions. The bond distances and bond angles are as follows: Zn–N 204 pm, Cd–N and Cd–S 225 and 265 pm, respectively, and Hg–S 254 pm, and Zn–N–C 145°, Cd–N–C and Cd–S–C 149° and 106°, respectively, and Hg–S–C 102°. The Raman bands of the C–S vibration within the tetrathiocyanato complexes of Zn(II), Cd(II), and Hg(II) ions in aqueous solutions appeared at 821 and 710 cm−1 for the Zn(II) and Hg(II) complexes, respectively, consistent with the values in the literature. The C–S stretching Raman bands of tetrathiocyanatocadmate(II) complex were resolved into two components at 779 and 732 cm−1. The C–S frequency shifted toward the higher frequency side than that (747 cm−1) of the free SCN− ion in the case of the binding of SCN− to metal ion via the N atom in the Zn(II) and Cd(II) cases, while the C–S bands influenced by the M–S bonding appeared on the lower frequency side in the Cd(II) and Hg(II) complexes. The 13C chemical shifts of the tetrathiocyanato complexes in aqueous solutions showed the downfield and upfield shifts for Zn(II) and Hg(II), respectively, and the intermediate for Cd(II). The nitrogen-14 resonance in [M(SCN)4]2− in aqueous solutions shifted upfield (−30.8 ppm) for Zn(II), downfield (30.3 ppm) for Hg(II), and in between for Cd(II) (−1.3 ppm). The trend of the 13C- and 14N-chemical shifts supported the different types of binding of ambient SCN− ion deduced from the X-ray and Raman measurements.