Multinuclear magnetic resonance study of zinc(II) and cadmium(II) complexing with isothiocyanate

A study of zinc(II) and cadmium(II) complexes with isothiocyanate ion has been completed, using a low-temperature, multinuclear magnetic resonance technique that permits the observation of separate resonance signals for bound and free ligand, and Cd(II) metal ion. The Zn2+-NCS- complexes were studied by H-1, C-13, and N-15 NMR spectroscopy. In the H-1 spectra, the intensity of the coordinated water signal, corresponding to a Zn(II) hydration number of six in the absence of NCS-, decreases dramatically as this anion is added, indicating the complexing process involves more than a simple 1:1 ligand replacement. The C-13 and N-15 NMR spectra reveal signals for four species, most reasonably assigned to a series of tetrahedrally coordinated Zn2+-NCS- complexes. In the Cd2+-NCS- solution spectra, the C-13 and N-15 signals for four complexes also are observed and they are three line patterns, corresponding to a doublet from Cd-113 J-coupling, and a dominant central peak, resulting from bonding to magnetically inactive Cd isotopes. The Cd-113 spectra, showing signals for four complexes, correlate well in all respects with the C-13 and N-15 results, including coupling in specific cases. The spectral results for both metal ions reflect binding at the nitrogen atom of NCS-, with the complexes changing from an octahedral to a tetrahedral configuration when doing so. Confirming evidence for these conclusions also was provided by several infrared measurements of these metal-ion systems.