Group 15 Zintl ions: A brief introduction

Zintl cluster anions were discovered in 1891 by Joannis, who observed that metallic sodium reacted with excess lead in liquid ammonia to form an intensely-coloured green solution. The anionic species responsible for the green colour was identified as Pb₉^4– by the German chemist Eduard Zintl by potentiometric titrations in the 1930s. Sn₉^4–, Sb₇^3– and As₇^3– were also identified by this method. It was later found that solutions of Zintl clusters could also be obtained by dissolving pre-formed precursor alloys of the post-transition elements and alkali metals in liquid ammonia or ethylenediamine. These alloys, known as Zintl phases, have the general formula M_xE_y (M = alkali metal; E = p-block element), and are produced by heating stoichiometric mixtures of the elements for several days, under an inert atmosphere. In solution xM⁺ and E_y^x– are formed, and this methodology has led to the discovery of a huge variety of Zintl clusters. These include the group 14 cluster anions E₅^2–, E₉^4–, E₉^3– and E₉^2– (E = Si, Ge, Sn, Pb); the group 15 cluster anions E'₇^3– and E'₁₁^3– (E' = P, As, Sb); and the Zintl ions of the heavier group 15 elements E''₄^2– (E'' = Sb, Bi). 

Zintl ions: A spotter's guide. Negatively charged atoms are shown in red.

My research is focussed on the group 15 Zintl ions of general formula E₇^3– (E = P, As, Sb). These clusters are electron-precise, meaning they possess exactly the right number of electrons to account for the bonding and the charge. They have a very similar shape to the organic molecule nortricyclane (C₇H₁₀), as E and E^– possess the same number of electrons as the CH and CH₂ fragments, respectively. Their structure consists of a three-membered ring of basal E atoms (E5, E6 and E7 in the picture below), three negatively charged two-coordinate bridging E atoms (E2, E3 and E4), and a single three-coordinate E atom at the apex (E1). 

E₇^3–, my favourite Zintl.

The reactivity of group 15 Zintl ions towards a variety of compounds has been studied. These studies have shown that the E₇^3– cage can either be retained in the product or fragment. If the E₇^3– cage is retained, it can exhibit three different bonding modes: η¹ (two-electron donating), in which the cluster bonds to an atom via one of its two-coordinate atoms; η² (four-electron donating), in which the cluster bonds via two of its two-coordinate atoms; and η⁴ (six-electron donating), in which the cluster bonds via two of its two-coordinate atoms and also via the two basal atoms bonded to these two-coordinate atoms. For η⁴-coordination to occur, the bond between the two basal atoms involved in coordination has to break.

P₇[FeCp(CO)₂]₃ (η¹-coordination to each Fe), [P₇PtHPPh₃]^2– (η²-coordination) and [P₇Cr(CO)₃]^3– (η⁴-coordination) (refs. 1-3).

It is also possible for fragmentation of the E₇^3– cage to occur, and this results in the formation of larger heteroatomic cluster alloys, some examples of which are shown below.

[NbSb₈]^3–, [Sb₇Ni₃(CO)₃]^3–, [As@Ni₁₂@As₂₀]^3– and [Ni₅Sb₁₇]^4–, all formed via fragmentation of E₇^3– (refs. 4-7).

No doubt this brief introduction has left you dying to know more about group 15 Zintl ions! Fear not, I hope to shortly post a review of all the research carried out so far. I haven't synthesised any of the molecules pictured in this post, but I will be writing about my own research in due course. Thanks for reading!

References

1. Ahlrichs, R.; Fenske, D.; Fromm, K.; Krautscheid, H.; Krautscheid, U.; Treutler, O., Chemistry-a European Journal 1996, 2 (2), 238-244.

2. Charles, S.; Fettinger, J. C.; Bott, S. G.; Eichhorn, B. W., Journal of the American Chemical Society 1996, 118 (19), 4713-4714.

3. Charles, S.; Eichhorn, B. W.; Rheingold, A. L.; Bott, S. G., Journal of the American Chemical Society 1994, 116 (18), 8077-8086.

4. Kesanli, B.; Fettinger, J.; Scott, B.; Eichhorn, B., Inorganic Chemistry 2004, 43 (13), 3840-3846.

5. Charles, S.; Eichhorn, B. W.; Bott, S. G., Journal of the American Chemical Society 1993, 115 (13), 5837-5838.

6. Moses, M. J.; Fettinger, J. C.; Eichhorn, B. W., Science 2003, 300 (5620), 778-780.

7. Moses, M. J.; Fettinger, J. C.; Eichhorn, B. W., Inorganic Chemistry 2007, 46 (4), 1036-1038.