MULTINUCLEAR MAGNETIC-RESONANCE STUDY OF ALUMINUM(III)-ISOTHIOCYANATE COMPLEXES IN WATER-ACETONE MIXTURES

A multinuclear magnetic resonance (NMR) study of the complexes of aluminum(III) with isothiocyanate ion in water-acetone mixtures has been completed. At temperatures low enough to slow proton and ligand exchange, separate resonance signals are observed for coordinated and bull H2O (H-1) and NCS- (C-13, N-15), and Al3+ (Al-27) in each complex. The H-1 NMR spectra reveal six sets of signals for the complexes, [Al(H2O)(6)](3+) through [Al(H2O)(NCS)(5)](2-), including isomers for three of the species. Signal area measurements show a decrease in the Al3+ hydration number with increasing NCS- concentration, as this anion replaces water in the solvation shell In the Al-27 NMR spectra of these systems, signals for seven complexes, [Al(H2O)(6)](3+) through [AI(NCS)(6)](3-), are observed, with chemical shifts increasing by about 6 ppm with each additional NCS-. Although broadened somewhat by the Al(III) quadrupole, the C-13 and N-15 NMR spectra also reveal coordinated NCS- signals for these complexes, including Al-27-(NCS)-C-13 J-coupling in [Al(NCS)(6)](3-). Area evaluations of the N-15 NMR signals provide an excellent complement to the H-1 hydration number data. These NMR results demonstrate that a multinuclear approach to the study of solution complexes can provide detailed structural information about the species being formed.