⁷⁷Se Solid-State NMR Spectroscopy and Structures of Tetramethylammonium Pentaselenide and Hexaselenide Complexes

High-resolution ⁷⁷Se solid-state NMR spectra are reported for [NMe₄Se₅ and [NMe₄]₂Se₆. The isotropic chemical shifts for the former are similar to those observed for the Se₅²⁻ 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 Se₆²⁻ anion in the solid state and solution is less good. The spectra show that the five selenium sites are distinct in [NMe₄]₂Se₅ while the symmetry of [NMe₄Se₆ is such that there are only three crystallographically different selenium sites. Single-crystal X-ray structure determinations of [NMe₄]₂Se₅ and [NMe₄]₂Se₆ are reported which are consistent with this observation. Using a, ß, and ? to signify the selenium atom positions relative to the ends of the chain, the shielding anisotropies for the a and ß sites are large (in the range 720–1175 ppm), while those for the ? sites are lower than these (614 and 332 ppm).