DIRECT H-1, C-13, AND N-15 NMR-STUDY OF MAGNESIUM(II)-ISOTHIOCYANATE COMPLEXING

A hydrogen-1, carbon-13, and nitrogen-15 NMR study of magnesium(II)-is othiocyanate complexation in aqueous mixtures has been completed. At t emperatures low enough to slow proton and ligand exchange, separate H- 1, C-13, and N-15 NMR signals are observed for coordinated and bulk wa ter molecules and anions. The H-1 NMR spectra reveal signals for the h exahydrate and the mono-through triisothiocyanato complexes, as well a s two small signals attributed to [Mg(H2O)(5)(OH)](1+) and [Mg(H2O)(4) (OH)(NCS)]. Accurate hydration numbers were obtained from signal area integrations at each NCS- concentration. in the N-15 NMR spectra, sign als also were observed for the mono-through triisothiocyanato complexe s, and a small signal believed to be due to [Mg(H2O)(4)(OH)(NCS)]. Coo rdination number contributions for NCS- were measured from these spect ra and when combined with the hydration numbers they totalled essentia lly six at each anion concentration. Signals for [Mg(H2O)(5)(NCS)](1+) through [Mg(H2O)(3)(NCS)(3)](1-) also were observed in the C-13 NMR s pectra and the area evaluations were comparable to the N-15 NMR result s. An analysis of the magnitude and sign of the coordinated NCS- chemi cal shifts identified the nitrogen atom as the anion binding site. All spectra indicated [Mg(H2O)(5)(NCs)](1+) and [Mg(H2O)(4)(NCS)(2)] were the dominant isothiocyanato complexes over the entire range of anion concentrations. The inability to detect evidence for complexes higher than the triisothiocyanato reflects the competitive binding ability of water molecules and perhaps the decreased electrostatic interaction b etween NCS- and negatively charged higher complexes.