solutions of metals in liquid ammonia

Here is one of the most beautiful and startling sights in inorganic chemistry. Dissolve a piece of sodium or potassium not in water (which would explode) but in cold liquid ammonia, and instead of a colourless salt solution you get an intense, deep blue liquid. Add more metal and the blue deepens, then suddenly the liquid turns into a bronze, metallic, lustrous phase. A metal has dissolved without reacting, and its electrons are floating free.

What happens is remarkable: the alkali metal ionizes, but the ammonia cannot stabilize a hydride-like product. Instead, the metal cation is solvated by ammonia molecules, and the freed electron is also solvated — trapped in a cavity surrounded by the positive ends of ammonia molecules. This solvated electron is a genuine free electron held in the liquid, and it is what absorbs light to give the brilliant blue colour (the same colour regardless of which metal you used, because the colour comes from the electron, not the cation). At higher concentrations the electrons pool together and the solution conducts electricity like a liquid metal, taking on the bronze sheen.

These solutions matter both as a window onto the solvated electron — one of the simplest reducing agents imaginable, a lone electron ready to be donated — and as practical reagents: dissolving-metal reductions in liquid ammonia (such as the Birch reduction) are standard tools in synthesis. The honest caveat: the solutions are metastable. Given time, or a catalyst such as a transition-metal surface, the solvated electrons slowly reduce the ammonia itself to give the amide and hydrogen gas, so the lovely blue fades — it is a thermodynamically downhill but kinetically slow decay, not permanent.