Octahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 array. One important class of octahedral clusters are chalcohalide

Chalcohalide

The chalcohalides are binary molecules formed of chemical elements taken from the chalcogens and the halides .-Examples:*Sulfur chloride*Oxygen fluoride*Tellurium chloride*Selenium chloride...

 clusters of the type M₆X₈L₆ where M is a metal usually of group

Periodic table group

In chemistry, a group is a vertical column in the periodic table of the chemical elements. There are 18 groups in the standard periodic table, including the d-block elements, but excluding the f-block elements....

 6 or group 7, X is a ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 and more specifically an inner ligand of the chalcohalide

Chalcohalide

The chalcohalides are binary molecules formed of chemical elements taken from the chalcogens and the halides .-Examples:*Sulfur chloride*Oxygen fluoride*Tellurium chloride*Selenium chloride...

 group such as chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 or sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 and L is an "outer ligand." The metal atoms define the vertices of an octahedron

Octahedron

In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....

. The overall point group symmetry

Molecular symmetry

Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

 is O_h. 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 Re₆S₈Cl₆^4- anion and the W₆I₁₄^2- anion, the last one by reaction of tungsten hexacarbonyl

Tungsten hexacarbonyl

Tungsten hexacarbonyl is the chemical compound with the formula W6. This complex gave rise to the first example of a dihydrogen complex....

 with iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 at elevated temperatures. Molybdenum(II) chloride

Molybdenum(II) chloride

Molybdenum dichloride describes chemical compounds with the empirical formula MoCl2. At least two forms are known, and both have attracted much attention from academic researchers because of the unexpected structures seen for these compounds and the fact that they give rise to hundreds of...

 is an example of a polymeric octahedral cluster; it reacts with chloride sources to afford Mo₆Cl₁₄^2-.

A well-studied class of solid state compounds related to the chalcohalides are molybdenum

Molybdenum

Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 clusters of the type A_xMo₆X₈ with X sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 or selenium

Selenium

Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 and A_x 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

Periodic table group

In chemistry, a group is a vertical column in the periodic table of the chemical elements. There are 18 groups in the standard periodic table, including the d-block elements, but excluding the f-block elements....

 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 Ta₆Cl₁₈^4-., Nb₆F₁₅ or the Nb₆F₁₈^2- anion.

Electron counting in octahedral clusters

The species Mo₆Cl₁₄^2- 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 Ta₆Cl₁₈^4- have fewer d-electrons. For example, the naked cluster Ta₆⁺¹⁴, the core of Ta₆Cl₁₈^4- 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.