The Arachno‐Zintl Ion (Sn$_{5}$Sb$_{3}$)$^{3-}$ and the Effects of Element Composition on the Structures of Isoelectronic Clusters: Another Facet of the Pseudo‐Element Concept

he pseudo‐element concept, in its most general formulation, states that isoelectronic atoms form equal numbers of bonds. Hence, clusters such as Zintl ions usually retain their structure upon isoelectronic replacement of some or all atoms. Here, a deviation from this common observation is presented, namely the formation of (Sn$_{5}$Sb$_{3}$)$^{3-}$ (1), a rare example of an eight‐vertex Zintl ion, and an unprecedented example of a Zintl ion synthesized by solution means that has an arachno‐type structure according to the Wade–Mingos rules. Three structure‐types of interest for (Sn$_{5}$Sb$_{3}$)$^{3-}$ were identified by DFT calculations: one that matched the X‐ray diffraction data, and two that that were reminiscent of fragments of known clusters. A study on the isoelectronic series of clusters, (Sn$_{x}$Sb$_{8-x}$)$^{2-x}$ (x=0–8), showed that the relative energies of these three isomers vary significantly with composition (independent of electron count) and that each is the global minimum at least once within the series.