Octahedral cluster

Octahedral clusters are inorganic or organometallic cluster compounds composed of six metals in an octahedral array.^[1] 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 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 with iodine at elevated temperatures. Molybdenum(II) chloride is an example of a polymeric octahedral cluster; it reacts with chloride sources to afford Mo₆Cl₁₄^2-.

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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, 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.^[2]

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 Ta₆Cl₁₈^4-.^[3], Nb₆F₁₅ or the Nb₆F₁₈^2- anion ^[1].

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.

References

1. ^ ^{a b} Eric J. Welch and Jeffrey R. Long Atomlike Building Units of Adjustable Character: Solid-State and Solution Routes to Manipulating Hexanuclear Transition Metal Chalcohalide Clusters in Progress in Inorganic Chemistry, Volume 54 Kenneth D. Karlin ISBN 0-471-72348-7 2005 Link
2. ^ Saito, T. and Imoto, H., "Chalcogenide Cluster Complexes of Chromium, Molybdenum, Tungsten, and Rhenium", Bulletin Chemical Society of Japan., 1996, volume 69, pp. 2403-2417.
3. ^ Koknat, F. W.; Marko, D. J. "Tetradecachlorohexatantalum Octahydrate, Ta₆Cl₁₄^.8H₂O" Inorganic Syntheses, 2004, volume 34, pp. 187-191. ISBN 0-471-64750-0. (describes Na₄Ta₆Cl₁₈)