Polyanionic Clusters and Networks of the Early p-Element Metals in the Solid State: Beyond the Zintl Boundary

Reduction of p-element (post-transition) metals and metalloids by alkali metals leads to many salts containing polyatomic clusters or network anions of these elements. The earliest solvated examples were referred to as Zintl ions. Synthetic explorations have now established that many of the clusters can in fact be obtained from neat (solvent free) high-temperature reactions of binary to quaternary systems, particularly for the heavier tetrel (group 14) and triel (group 13) elements. Some synthetic tricks have also proven useful. Electronic guidelines such as Wade's rules, known to account well for other types of electron-deficient cluster bonding, are widely applicable to these compounds, but numerous hypoelectronic (electron-poor) trielide salts have also been discovered. These developments also extend to related infinite network structures and Zintl (valence) compounds. The Zintl boundary designation traditionally delineated the tetrel elements that form salts with the active metals from those of the triel and earlier elements that were once thought to generate only intermetallic phases. The distinction no longer seems appropriate, at least with regard to some alkali-metal compounds of the triel elements.