INTERSTITIALLY-STABILIZED CLUSTER-BASED HALIDES OF THE EARLY TRANSITION-METALS

A large number of reduced halides of the first two groups of transition metals are obtained that are built from edge-bridged octahedral cluster units (M(6)X(12)) but only when these contain a centered (interstitial) heteroatom Z. The third component provides not only central bonding to the M(6)X(12) cluster but also valence electrons that aid in fulfilling certain minimal electron counts necessary in clusters of these electron-poor metals. Recent results are surveyed for zirconium and the rare-earth metals (group 3 plus the lanthanides). Much of the versatility and beauty of zirconium cluster phases originate with the diverse ways for intercluster bridging by additional halide plus a choice of 15 examples of Z. Some new tunnel, perovskite derivative, and network structures, as well as bonding and electronic regularities are described for these. Rare-earth metal examples afford substantial contrasts with only a few ternary examples that contain isolated clusters, these also being hypostoichiometric with fewer than 12X per 6M. Other phases formed contain either tetrameric oligomers or infinite chains constructed from classical clusters that have been condensed through sharing of metal edges. The rare-earth element examples are noteworthy in their ability to bind 18 different Z, which include 13 of the 3d, 4d or 5d metals as the interstitial component. Some relevant properties are also described.