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METAL CLUSTERS The oldest identified metal cluster is Calomel which was known to the India ns in the 12th century. The existence of a Mercury to mercury bond in this compound was established in beginning of the 20th century. Metal cluster are prominently found with Refractory Metals . In general metal centers with large overlapping D-orbital s form stable clusters. Thus metals with a high Oxidation State and therefore large effective charge tend to be unstable. Polynuclear Metal Carbonyl s are generally found in late Transition Metal s with low formal Oxidation States . Polynuclear Halide s and Oxide s are found with early transition metals. An often studied metal cluster compounds is dinuclear Potassium Octachlorodirhenate(III) or K2Re2Cl8 whose peculiar molecular structure is explained by Quadrupole Bonding . Another dinuclear compound is Di-tungsten Tetra(hpp) , the currect record holder low Ionization Energy . In trinuclear rhenium(III) chloride or (ReCl3)3 the metal centers bond directly and through chlorine bridges. Because this compound is Diamagnetic and not Paramagnetic the Rhenium bonds are Double Bond s and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in Hydrochloric Acid a Re3Cl123- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W4(OCH3)12 with Tungsten single bonds and molybdenum chloride (Mo6Cl8)Cl4 is a hexanuclear Molybdenum compound and an example of an Octahedral Cluster . A special group of clusters with the general structure MxMo6X8 such as PbMo6S8 form a Chevrel Phase which exhibit Superconductivity at low temperatures. So-called Zintl Compound s form a separate class of metal clusters. they form when Poor Metal s are dissolved in liquid Ammonia in the presence of Alkali Metal s. Examples of Zintl anions are Bi33-,Sn94-,Pb74- and Sb73-. These anions do not require ligands and called naked clusters but are unstable nevertheless when not stabilzed as Salt s by Cryptate s. In certain metal clusters Metal Aromaticity is observed. SEE ALSO
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