Octahedral cluster

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.

Octahedral chalcogenide and halide clusters

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 L_i, for inner (innen in the original German description), and those ligands attached only to one metal are labeled outer, or L_a for ausser. Typically, the outer ligands can be exchanged whereas the bridging ligands are more inert toward substitution.

Face-capped halide clusters

The premier example is of the class is Mo₆Cl₁₄^{2−}. 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 W₆Cl₁₄^{2−} anion, which is obtained by extraction of tungsten(II) chloride.

Chalcohalide clusters

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 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.

Chevrel clusters

A well-studied class of solid-state compounds related to the chalcohalides are molybdenum clusters of the type A_xMo₆X₈ with X sulfur or selenium and A_x 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 Ã‚°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 Nb₆F₁₈^{2−}.

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

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 Ta₆¹⁴⁺, 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.

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.

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