High-valence cluster ligands

In the high-valence cluster species, metal atoms forming the metal network appear to have positive intermediate oxidation states. However, metal oxidation states in cluster are always lower than those characteristic for the same metals in classical mononuclear complexes. The ligands associated to this class of cluster are normally good cr-donors that according to the classification of Pearson would have intermediate hardness. Among these compounds the most frequent are the halides, specially chlorides and bromides. An example of this cluster class is the molybdenum species [Mo6Xs] shown in Fig. 2.1c. 

There are two large classes of molecular clusters in which the ligands are all of the same nature The halide cluster class and the carbonyl or, in general, organometallic cluster class. According to the formal oxidation numbers acquired by metal atoms in these two classes of clusters, they are often classified as high and low-valence clusters respectively. 

Ligand Exchange in Metal Halide Clusters. Rhenium, molybdenum, tantalum and niobium halides are high-valence metal clusters that formally behave as Lewis acids towards halide ions forming anionic complexes that retain the structure of the original cluster. 

The low number of valence orbitals available for cluster-ligand bonding commented on above in addition to the high reduction potentials of alkali metals cause alkali metal clusters to be rather unstable. Disproportionation into the metal and other products with the element in higher oxidation states is often a thermodynamic favorable process. 

---