Werner cluster complexes

Thus, the stmctural chemistry of reduced rare-earth metal cluster complexes may be described in a way similar to silicates—where [SiOJ tetrahe-dra share common vertices—or like (anti-)Werner complexes that are either monomers, oligomers, or developed into coordination polymers of various dimensionalities. 

For practical purposes the field of metal-metal bonds and metal atom clusters can be divided into two broad areas. (1) Those compounds with the metal atoms in formal oxidation states of zero or close to it, including negative ones. For the most part these are polynuclear metal carbonyls, or very similar compounds. In these compounds the M-M bonds are usually long, weak and of order one. (2) Compounds with the metal atoms in low to medium positive oxidation states, and ligands of the same kinds normally found in classical Werner complexes, e.g., halide, sulfate, phosphate, carboxylate or thiocyanate ions, water, amines and phosphines. Compounds of this type include metal-metal bonds of orders ranging from about 1/2 to 4.0. 

Those in which the metal atoms are in somewhat higher oxidation states (+2 to +4) and the ligands are typically halide, sulfide, or oxide ions and some others of the same ilk as those in mononuclear Werner complexes. Clusters of this type are most common among the early transition elements, groups 5-7. 

There are two main classes of cluster compounds — those where the metal atoms have a low formal oxidation state, in which case the ligands are almost always CO groups and those with metals that have a high oxidation state, ranging from +2 to +4. In this case, the ligands are usually halides, sulfides, oxide ions, or ligands from a monomolecular Werner complex (see Chapter 9 for details on a Werner complex). 

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