A Molecular Perspective on Lithium–Ammonia Solutions

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 to 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 to 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.

Surely the earliest observations of the spectacular blue 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 Davys 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. 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—had lifetimes of at least a few seconds. More than 50 years after Davy, W. Weyl independently made similar observations on the solubility of alkali metals in liquid ammonia. 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 smademetal ammoniums”; that is compounds in which one or more of the hydrogen atoms in NH4 is replaced by a metal atom. Metal–ammonia solutions were usually referred to as metal ammoniums well into the twentieth century. The study of metal–ammonia solutions was materially advanced by W. Seely in 1871 who gave the first clear and forthright recognition of the “nonchemical” solvent action of ammonia, based on his own experiments, and corrected what he termed as Weyls “imperfect comprehension of the fundamental facts”, pointing out that “the blue liquid is a simple solution of Na in ammonia”. Seely further noted: “I mean that these metals dissolve in the ammonia as salt dissolves in water”. During the first part of the twentieth century, E. C. Franklin and C. A. Kraus probably did more to elucidate the chemistry of liquid ammonia solutions than everybody else combined. It is perhaps little known that their work was prompted by the research and insight of H. P. Cady, carried out while he was an undergraduate! Whilst working on cobalt ammine complexes, Cady proposed that ammonia in these (and other “double salts”) must function in a manner akin to water in salts with water of crystallization. He suggested further that liquid ammonia would probably be found to resemble water in its physical and chemical properties—thus adding a second to our list of ionizing solvents. Cadys undergraduate work, carried out without supervision, published in 1897, was perhaps the first physical chemistry study of liquid ammonia solutions. It was Kraus and co-workers, in truly classic studies over a period of almost half a century, who obtained the quantitative data on the electrical conductance and other physical properties which has formed much of the experimental basis for the development and testing of various models for these solutions. Kraus recognized that solutions of the alkali metals (as well as the alkaline earth metals) are truly unique; they constitute a direct link between electrolytes, on the one hand, and metals on the other.

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