Raman Intensity Study of Zn(II), Cd(II), and Hg(II) Thiocyanate Complexes in Aqueous Solutions

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Abstract
Raman spectra of the tetrathiocyanato complexes of Zn(II), Cd(II), and Hg(II) were measured in aqueous solution. Polarized Raman lines appear for SCN− at 747 and 2068 cm−1, for Zn(NCS)42− at 153, 817, and 2110 cm−1, for Cd(CNS)42− at ∼747 and 2098 cm−1, and for Hg(SCN)42− at 236, 717, and 2114 cm−1. The shift of the C–S frequency on complex formation is consistent with the binding of NCS− to Zn(II) through the N, to Hg(II) through the S, and to Cd(II) with some NCS− turned each way. Support for the proposed ambidentate binding to Cd(II) is given by the appearance in solutions having lower SCN− to Cd(II) ratios of lines characteristic of lower complexes showing separate frequencies characteristic of both S‐bound and N‐bound NCS−. Intensities of the various Raman lines were analyzed to give bond derived polarizabilities, which further support the different types of binding. Bonding of N to Zn causes a lowering of the C–N bond derived polarizability and an increase in that of the C–S. Strong sulfur binding by Hg(II) leads to a large value for the C–N bond derived polarizability while the value of this derivative for the Cd complex is intermediate between those of the Zn and Hg complexes and approximates that of free thiocyanate. The value of the derived polarizability for the C–S stretch of Hg complex is larger than is anticipated, but the magnitude in the Cd complex is identical with that of free thiocyanate.