The rare-earth elements have essentially three valence electrons; the 4f electrons may be treated as core electrons. The divalent state comes in two varieties, with electronic configurations of [Xe]4fn5d06s0 or [Xe]4fn-15d16s0. The first gives rise to alkaline-earth-metal salt-like behavior; in the latter case the 5d1 electron may either be delocalized into a 5d (conduction) band (solid state) or trapped in an anion with superbulky ligands (organometallics). Furthermore, 5d orbitals play a decisive role in the formation of rare-earth metal (R) clusters encapsulating an endohedral transition metal atom (T). These clusters may be isolated─although not naked─or condensed via common edges (frequent), faces (rare) or vertices (extremely rare) to oligomers, chains, layers or networks. All of these polar-intermetallic clusters are built into solids with the addition of halide ligands (X). Bonding in these cluster complexes is predominantly heteroatomic, T─R and R─X, respectively, with little, nevertheless important, homoatomic T─T (if possible) and R─R interactions.

To cluster or not to cluster

249th National Meeting (2015)

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