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 The species Mo6Cl142− feature Mo(II) (d4) centers. Six Mo(II) centers gives rise to a total of 24 valence electrons, or 2e/Mo-Mo vector. More electron-deficient derivatives such as Ta6Cl184− have fewer d-electrons. For example, the naked cluster Ta614+, the core of Ta6Cl184− would have 5(6) - 14 = 16 valence electrons. Fewer d-electrons result in weakened M-M bonding and the extended Ta---Ta distances accommodate doubly bridging halides. Other classes of octahedral clusters In the area of metal carbonyl clusters, a prototypical octahedral cluster is [Fe6C(CO)16]2−, which is obtained by heating iron pentacarbonyl with sodium. Some of the CO ligands are bridging and many are terminal. A carbide ligand resides at the center of the cluster. A variety of analogous compounds have been reported where some or all of the Fe centres are replaced by Ru, Mn and other metals. Outside of carbonyl clusters, gold forms octahedral clusters. References Cluster chemistry