ChemInform Abstract: Exploring the Metal-Rich Chemistry of the Early Transition Elements

The early transition metals, especially Zr, Hf, Hb and Ta, exhibit a metal-rich chemistry that is often surprising in its structural and physical aspects. Unfamiliarity with this chemistry is illustrated by the discovery of several new binary compounds in the Ta-S, Ta-Se, Ta-Te, and Hf-Te systems within the past few years. Some striking differences observed between the metal-rich chalcogenides of Zr and Hf or between Nb and Ta challenge basic presumptions about the similarity of these congeneric pairs. The factors controlling the structural anisotropy of a new class of tetragonal layered compounds that includes Ta 2 Se, Ta 2− x Nb x S, Hf 3 Te 2 , and ZrZTe (Z  Si, Ge, Sn) are discussed. Strongly early-late transition intermetallic bonding leads to the formation of an expanding class of compounds that includes Ta 9 M 2 S 6 (M  Fe, Co, Ni), Ta 11 M 2 Se 8 (M  Fe, Co, Ni), Ta 8 NiSe 8 and the newly discovered hafnium tellurides, Hf 8 MTe 6 (M  Mn, Fe, Co, Ni, Ru) and Hf 5 MTe 3 (M  Fe, Co). Our efforts to dismantle solid-state Zr-halide cluster compounds is described. Ambient temperature molten salts help us achieve the controlled excision of [(Zr 6 Z)CL 18 ] n− from solid state precursors; we describe the applications of electronic and NMR spectroscopies in characterizing clusters in solution. Finally, we discuss bonding in metal-rich systems, with particular emphasis on localized bonding descriptions for metalmetal bonds in extended metal-linked networks. Such localized descriptions increase our understanding of otherwise anomalous properties and illuminate the artificiality of separate “metallic” and “covalent” bonding concepts.