text: As highly reduced species in solution, Zintl ions offer many and often unexpected, reaction possibilities, and their discrete nature positions them as potentially important starting materials in inorganic synthesis.
  In solution, individual Zintl ions can react with each other to form oligomers and polymers. In fact, anions with high nuclearity can be viewed as oxidative coupling products of monomers. After oxidation, the clusters may sometimes persist as radicals that can be used as precursors in other reactions. Zintl ions can oxidize without the presence of specific oxidizing agents through solvent molecules or impurities, for example in the presence of cryptand, which is often used to aid crystallization.
  Zintl ion clusters can be functionalized with a variety of ligands in a similar reaction to their oligomerization. As such, functionalization competes with those reactions and both can be observed to occur. Organic groups, for example phenyl, TMS, and bromomethane, form exo bonds to the electronegative main group atoms. These ligands can also stabilize high nuclearity clusters, in particular heteroatomic examples.
  Similarly in solids, Zintl phases can incorporate hydrogen. Such Zintl phase hydrides can be either formed by direct synthesis of the elements or element hydrides in a hydrogen atmosphere or by a hydrogenation reaction of a pristine Zintl phase. Since hydrogen has a comparable electronegativity as the post-transition metal it is incorporated as part of the polyanionic spatial structure. There are two structural motifs present. A monatomic hydride can be formed occupying an interstitial site that is coordinated by cations exclusively (interstitial hydride) or it can bind covalently to the polyanion (polyanionic hydride).
  The Zintl ion itself can also act as a ligand in transition metal complexes. This reactivity is usually seen in clusters composed of greater than 9 atoms, and it is more common for group 15 clusters. A change in geometry often accompanies complexation; however zero electrons are contributed from the metal to the complex, so the electron count with respect to Wade's rules does not change.  In some cases the transition metal will cap the face of the cluster. Another mode of reaction is the formation of endohedral complexes where the metal is encapsulated inside the cluster. These types of complexes lend themselves to comparison with the solid state structure of the corresponding Zintl phase. These reactions tend to be unpredictable and highly dependent on temperature, among other reaction conditions.