Metal clusters inside out.

Solid-state chemistry of cluster compounds with metals in the left part of the periodic table is described. As a function of the metal valence electron concentration (VEC), characteristic changes occur with stepwise changing features. Strongly metal-metal-bonded and ligand-encapsulated clusters exist for large values of VEC. At decreased values, endohedral atoms stabilize the clusters, and with the lowest values, only clusters exist with no ligand shell. This general trend holds for systems with discrete, as well as condensed, clusters as illustrated for d metals, including the lanthanides, alkaline earth and alkali metals.