2.6 Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity

One little-known area of chemistry is the chemistry of heavy main-group p elements, mostly metals and semimetals, in negative oxidation states. While the chemistry of Sn2+ and Sn4+, for example, is very well studied and we all learn about it very early in our education, not many people are aware of the fact that tin atoms can accept electrons and exhibit completely different reactivity when negatively charged. For example, catenation and clustering are uncommon for cationic tin, but is the norm for anions, both in inter-metallics and molecular compounds. Thus, cyclopentadienyl-like Sn6− 5 is found in the inter-metallic compounds Na8AeSn6 (Ae = Ba, Eu), Li9−x AeSn6+x (Ae = Ca, Eu), Li5Ca7Sn11, and Li6Eu5Sn9, isolated tetrahedra of Sn 4− 4 and alkali-metal cations constitute the structures of A4Sn4 (A = alkali metal),2 while Na4CaSn6 and Li2Ln5Sn7 (Ln = Ce, Pr, Sm, Eu) contain infinite anionic chains and heptane-like Sn16− 7 oligomers, respectively. 3,4 Notice that, as might be expected, the negative oxidation states are achieved when the more electronegative p element, tin in this case, is combined with much more electropositive s or f element such as the alkali, alkaline-earth, or rare-earth metals. Notice also that, for the purpose of structure rationalization and systematic description, the formal oxidation state assignments assume complete electron transfer from the more electropositive atoms to the p element. Such polar inter-metallic compounds, called Zintl phases when electronically balanced, are known for all post-transition metals and semimetals.5 Negatively charged clusters of some of the p-metals and semimetals are also stable in solutions and can be crystallized from them as ionic molecular compounds. Known, at present, are a number of such ligand-free anionic clusters, called Zintl ions.6 Some examples are the clusters with general formulas Pn3− 7 , E 3− 9 , E 4− 9 , and E 2− 5 where Pn = P, As, Sb and E = Si, Ge, Sn, Pb. This chapter will limit its attention to the synthesis, structure, and functionalization of molecular anionic clusters of tin. Although both nineand five-atom clusters are known (Figure 2.6.1), the focus will be predominantly on the nine-atom deltahedral clusters Sn3− 9 and Sn 4− 9 which are the most readily prepared and most extensively studied species.