Chevrel phase
Octahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral array. Many types of compounds are known, but all are synthetic.
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. Typically, the outer ligands can be exchanged whereas the bridging ligands are more inert toward substitution.
The premier example is of the class is Mo6Cl142−. This dianion is available as a variety of salts by treating the polymer molybdenum(II) chloride with sources of chloride, even hydrochloric acid. A related example is W6I142− anion, which is obtained by reaction of tungsten hexacarbonyl with iodine at elevated temperatures.
A related class of octahedral clusters are 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 chloride or sulfide 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.
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, called Chevrel phases or Chevrel clusters, have been actively studied because they are type II superconductors with relatively high critical fields. 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.
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