On the solvation of metal ions in liquid ammonia: a molecular simulation study of M(NH2)x(NH3)y complexes as a function of pH

Abstract

The solvation of mono-, di- and trivalent metal ions in liquid ammonia is characterized from molecular simulations. A central focus of the analyses is given by metal ion acidity within the respective solvent complexes. For this purpose, a recently developed model for estimating the pK of the auto-protolysis reaction 2NH₃ → NH₄⁺ + NH₂⁻ is transferred to assessing the ‘local’ pK of ammonia molecules coordinating a metal ion. On this basis, we identify M^I/II(NH₃)_n^+/++ complexes (M = Na⁺, K⁺, Mg²⁺ with n = 6 and M = Ca²⁺ with n = 8, respectively) as predominant species unless extremely ammono-basic conditions are imposed. On the other hand, the trivalent Al³⁺ and Ga³⁺ species were found to favor [Al(NH₂)₃(NH₃)₂]⁰ and [Ga(NH₂)₄]⁻ complexes, respectively. The negatively charged complexes dominate over a wide range of pH for gallium ion solvation in ammonia, whilst [Al(NH₂)₄(NH₃)₂]⁻ complexes require moderately ammono-basic solutions.