Alkali metals in ethylenediamine: A computational study of the optical absorption spectra and NMR parameters of [M(en) ₃ ^δ+̇M^δ-] ion pairs

The optical absorption spectra of alkali metals in ethylenediamine have provided evidence for a third oxidation state, -1, of all of the alkali metals heavier than lithium. Experimentally determined NMR parameters have supported this interpretation, further indicating that whereas Na^- is a genuine metal anion, the interaction of the alkali anion with the medium becomes progressively stronger for the larger metals. Herein, first-principles computations based upon density functional theory are carried out on various species which may be present in solutions composed of alkali metals and ethylenediamine. The energies of a number of hypothetical reactions computed with a continuum solvation model indicate that neither free metal anions, M ^-, nor solvated electrons are the most stable species. Instead, [Li(en)₃]₂ and [M(en) ₃ ^δ+̇M^δ-] (M = Na, K, Rb, Cs) are predicted to have enhanced stability. The M(en)₃ complexes can be viewed as superalkalis or expanded alkalis, ones in which the valence electron density is pulled out to a greater extent than in the alkali metals alone. The computed optical absorption spectra and NMR parameters of the [Li(en)₃] ₂ superalkali dimer and the [M(en) ₃ ^δ+̇M^δ-] superalkali-alkali mixed dimers are in good agreement with the aforementioned experimental results, providing further evidence that these may be the dominant species in solution. The latter can also be thought of as an ion pair formed from an alkali metal anion (M ^-) and solvated cation (M(en)₃⁺).