Eva Zurek, Peter P. Edwards, and Roald Hoffmann* Angewandte Chemie Keywords: ammonia · lithium · metal–nonmetal transitions · molecular orbitals · solvated electrons Dedicated to Professor Ron Catterall on the occasion of his 72nd birthday, and in memory of Mike (Michell) J. Sienko Reviews R. Hoffmann et al. 8198 www.angewandte.org 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 8198 – 8232 1.
Introduction 1.1. A Brief History Surely the earliest observations of the spectacular blue and bronze colors of alkali metal–ammonia solutions can be traced to the work of Sir Humphry Davy. Ammonia has been known for centuries, thanks to its biological origin. Davy, in 1807, first isolated potassium and then sodium. Thus, in 1807 and 1808 he had ammonia, potassium, and sodium available for his use. In his experiments to prove that potassium was indeed an element, and not a hydride of potash, he reacted grains of potassium with dry, gaseous ammonia to produce the beautiful colors of the concentrated, and then dilute films of potassium–ammonia. Ammonia was not liquefied until 1823 by Michael Faraday (at Davy’s suggestion), and hence Davy was seeing the dissolution of metallic potassium by dry gaseous ammonia. These observations were not published by Davy, but were uncovered by one of us in 1981 from a search of his laboratory notebooks.[1–3] The notebook entries are reproduced in Figure 1. They illustrate that 200 years ago Davy observed the characteristic visual signature of these solutions; their “fine blue colour”. Hannay and Hogarth in 1879 and 1880 also reported that gaseous ammonia dissolves metallic sodium (in essence, reproducing Davys observations on potassium and gaseous ammonia) and that these “gaseous solutions”—perhaps today they would be called cold plasmas[4]—had lifetimes of at least a few seconds.[5, 6] More than 50 years after Davy, W. Weyl independently made similar observations on the solubility of alkali metals in liquid ammonia.[7] This was the first description in the literature of what we now recognize as the preparation of a solution of a metal in liquid ammonia (actually made by the action of dry ammonia gas under pressure on potassium using a Faraday tube). Weyl, mistakenly, thought of these solutions as “metal ammoniums”; that is compounds in which one or more of the hydrogen atoms in NH4 is replaced by a metal A detailed molecular orbital (MO) analysis of the structure and electronic properties of the great variety of species in lithium– ammonia solutions is provided. In the odd-electron, doublet states we have considered: e@(NH3)n (the solvated electron, likely to be a dynamic ensemble of molecules), the Li(NH3)4 monomer, and the [Li(NH3)4 + · e@(NH3)n] ion-pairs, the Li 2s electron enters a diffuse orbital built up largely from the lowest unoccupied MOs of the ammonia molecules. The singly occupied MOs are bonding between the hydrogen atoms; we call this stabilizing interaction H H bonding. In e@(NH3)n the odd electron is not located in the center of the cavities formed by the ammonia molecules. Possible species with two or more weakly interacting electrons also exhibit H H bonding. For these, we find that the singlet (S = 0) states are slightly lower in energy than those with unpaired (S = 1, 2…) spins. TD–DFT calculations on various ion-pairs show that the three most intense electronic excitations arise from the transition between the SOMO (of s pseudosymmetry) into the lowest lying p–like levels. The optical absorption spectra are relatively metal–independent, and account for the absorption tail which extends into the visible. This is the source of Sir Humphry Davy’s “fine blue colour” first observed just over 200 years ago.
