Octahedral cluster

Octahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral array. One important class of octahedral clusters are chalcohalide clusters of the type M6X8L6 where M is a metal usually of group 6 or group 7, X is a ligand and more specifically an inner ligand of the chalcohalide group such as chlorine or sulfur and L is an "outer ligand." The metal atoms define the vertices of an octahedron. The overall point group symmetry is Oh. Each face of the octahedron is capped with a chalcohalide and eight such atoms are at the corners of a cube. For this reason this geometry is called a face capped octahedral cluster. Examples of this type of clusters are the Re6S8Cl64- anion and the W6I142- anion, the last one by reaction of tungsten hexacarbonyl with iodine at elevated temperatures. Molybdenum(II) chloride is an example of a polymeric octahedral cluster; it reacts with chloride sources to afford Mo6Cl142-. A well-studied class of solid state compounds related to the chalcohalides are molybdenum clusters of the type AxMo6X8 with X sulfur or selenium and Ax an interstitial atom such as Pb. These materials, commonly called Chevrel phase, have been actively studied because they are type II superconductors with relatively high critical fields. Such materials are prepared by high temperature reactions of the chalcogen and Mo metal. Related, soluble analogues have been prepared, e.g. Mo6S8(PEt3)6.

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. Example of this type of species are the anion Ta6Cl184-. , Nb6F15 or the Nb6F182- anion.

Electron counting in octahedral 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 Ta6+14, 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.