These compounds are bound together by metal-metal bonding as well as two kinds of ligands. Ligands that span the faces or edges of the M6 core are labeled Li, for
inner (innen in the original German description), and those ligands attached only to one metal are labeled outer, or La for
ausser. Such materials are prepared by high temperature (1100 °C) reactions of the chalcogen and Mo metal. Structurally related, soluble analogues have been prepared, e.g., Mo6S8(PEt3)6.
Edge-Capped Halide Clusters With metals in
group 4 or 5 a so-called
edge-capped octahedral clusters are more common. Twelve halides are located along the edge of the octahedron and six are terminal. Examples of this structure type are
tungsten(III) chloride, Ta6Cl14(H2O)4, Nb6F15, and Nb6F182−. Many of the early metal clusters can only be prepared when they incorporate interstitial atoms. One example is Zr6CCl12.
Tin(II) clusters Octahedral clusters of tin(II) have been observed in several solid state compounds. The reaction of tin(II) salts with an aqueous base leads to the formation of tin(II) oxyhydroxide (Sn6O4(OH)4), the structure of which comprises discrete Sn6O4(OH)4 clusters. In Sn6O4(OH)4 clusters, the six tin atoms form an octahedral array with alternate faces of the octahedron occupied by an oxide or hydroxide moiety, each bonded in a μ3-binding mode to three tin atoms. Crystal structures have been reported for compounds with the formula Sn6O4(OR)4, where R is an
alkoxide such as a methyl or ethyl group. Recently, it was demonstrated that anionic tin(II) clusters [Sn6O8]4- may form the close packed arrays as in the case of α-Sn6SiO8, which adopts the
zinc blende structure, comprising a face-centred-cubic array of [Sn6O8]4- clusters with Si4+ occupying half of the tetrahedral holes. A polymorph, β-Sn6SiO8, has been identified as a product of pewter corrosion in aqueous conditions, and is a structural analogue of
wurtzite. ==Electron counting in octahedral halide and chalcogenide clusters==