SE-77 SOLID-STATE NMR-SPECTROSCOPY AND STRUCTURES OF TETRAMETHYLAMMONIUM PENTASELENIDE AND HEXASELENIDE COMPLEXES

High-resolution Se-77 solid-state NMR spectra are reported for [NMe(4)]Se-2(5) and [NMe(4)]Se-2(6). The isotropic chemical shifts for the former are similar to those observed for the Se-5(2-) ion in solution, suggesting that the conformation adopted by the anion is similar in the solid state and solution. The agreement between the chemical shifts for the Se-6(2-) anion in the solid state and solution is less good. The spectra show that the five selenium sites are distinct in [NMe(4)]Se-2(5) while the symmetry of [NMe(4)]Se-2(6) is such that there are only three crystallographically different selenium sites. Single-crystal X-ray structure determinations of [NMe(4)]Se-2(5) and [NMe(4)]Se-2(6) are reported which are consistent with this observation. Using alpha, beta, and gamma to signify the selenium atom positions relative to the ends of the chain, the shielding anisotropies for the alpha and beta sites are large (in the range 720-1175 ppm), while those for the gamma sites are lower than these (614 and 332 ppm).