Cluster chemistry: Encyclopedia II - Cluster chemistry - Metal clusters

Cluster chemistry - Metal clusters

The oldest identified metal cluster is calomel which was known to the Indians 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-orbitals form stable clusters. Thus metals with a high oxidation state and therefore large effective charge tend to be unstable. Polynuclear metal carbonyls are generally found in late transition metals with low formal oxidation states. Polynuclear halides and oxides 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 bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a Re₃Cl₁₂^3- 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 a octahedral cluster. A special group of clusters with the general structure M_xMo₆X₈ such as PbMo₆S₈ form a Chevrel phase which exhibit superconductivity at low temperatures.

So-called Zintl compounds form a separate class of metal clusters. they form when poor metals are dissolved in liquid ammonia in the presence of alkali metals. Examples of Zintl anions are Bi₃^3-,Sn₉^4-,Pb₇^4- and Sb₇^3-. These anions do not require ligands and called naked clusters but are unstable nevertheless when not stabilzed as salts by cryptates.

In certain metal clusters metal aromaticity is observed.

Adapted from the Wikipedia article "Metal clusters", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki