Crystalline alkalides are salts with alkali metal anions (Na^-, K^-, Rb^-, or Cs^-) that have been known since 1974, when the first sodide was synthesized.1 The first crystalline electride, in which trapped electrons serve as the anions, was first fully characterized in 1986.2 The key to the formation of both classes of materials is complexation of the countercation (Li⁺ through Cs⁺) by organic macrocyclic or macrobicyclic molecules such as crown ethers and cryptands. Seven electrides and 37 alkalides have now been synthesized and their structures were determined by single crystal x-ray crystallography.3 Their properties were studied by a number of methods, including optical, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopy, magnetic susceptibility, conductivity, and differential scanning calorimetry (DSC).

Both alkalides and electrides are powerful reducing agents; so powerful, in fact, that most cannot survive, even in vacuo at temperatures above about - 30°C. The organic complexant is irreversibly destroyed by cleavage of the O-C or N-C bonds of the complexant. This reducing ability of alkalides is, however, useful in organic synthesis when a powerful two-electron reducing agent is needed.4 Alkalides and electrides in solution are able to reduce nearly all metallic cations to form nanoscale metal particles.5

In this overview of alkalides and electrides, the methodologies and special techniques used will be briefly described, and the structural features and electronic, optical and magnetic properties will be summarized. The reference list is far from complete, but the information given can provide many additional references to the original literature in this field.