Solid-State Cluster Chemistry - An Overview

Main group element clusters without ligands belong to another class of cluster compounds and can also be classified into discrete cluster ions and solid-state clusters. Some of their bonding properties are similar to those of the transition-metal clusters, and in addition to being important in their own right, information obtained from compounds such as Zintl phases is very useful for an overview of metal-metal bonding. 

Metal-cluster skeletons are defined by the number of metal cluster electrons (MCE) this in turn is determined by the positions of the metallic elements in the periodic table, and by their oxidation states. A general principle is the availability of vacant metal coordination sites for the formation of metal-metal bonds and of the valence eleetrons to be used in the formation of these bonds. Metals in relatively low oxidation states are, therefore, favored and the value of MCE corresponds to the number of M M bonds if electron-precise 2c 2e bonding is assumed. Several empirical rules have been established which describe the relationships between MCE and cluster shape. There is some variation in the modes by which the clusters are linked, depending on the oxidation states of the component metals and on the number of vacant coordination sites on metals. 

Seven contributions covering aspects of transition metal cluster chemistry and one concerning group-14 element clusters constitute this chapter on solid-state cluster chemistry. The general topics discussed are the structures of the cluster compounds and the electronic bonding schemes used to describe them. The importance of the bridges between the molecular units which form the solid-state cluster structures should be emphasized because of the general treatments of the electronic structures by use of modified molecular orbital calculations that are based on discrete cluster units. 

The structural chemistry described in Section 5.3 is concerned with reduced niobium oxides in which various NbeOu clusters are discernible. - Phases with ordered structures have been prepared by careful choice of preparative conditions. Reduced niobium oxides have NbeOn cluster cores which are discrete (OD) or linked together through apical oxygen atoms to form ID (chain), 2D (layer), or 3D 

For the corner-sharing NbeOn cluster units, the number of electrons for the different kinds of Nb atoms has been assigned. Generally a magic number of ca. 14 metal-cluster valence electrons are available for the M-M bonds in the NbgOn clusters. EH band calculations have been performed to assign the nature of the valence electrons. 

---