The history of the rare earths has entered its third century; trans
uranium elements are now a half century old. Both the lanthanide and
actinide ele- ments, 30 elements altogether, are f elements, meaninj
that their metallic 2 1 1 electronic configurations are typically 6s 5d
4f" and 7s 6d 5f" respectively. To an elementary approximation as
summarized in the 'average inorganic chemistry textbook, these
configurations cause their chemistry to be described by the trivalent
state accompanied by less interesting effects such as the lanthanide
contraction. However, the discovery of divalent and tetravalent
lanthanides and di- to seven-valent actinides hinted at the existence of
more interesting although still classic solid-state and coor- dination
chemistry. Metallic halides and chalcogenides and electron-poor cluster
compounds have been the outgrowth of many synthetic efforts during the
past 25 years or so. These days, one can say that the lan- thanides and
actinides are not at all boring; the fascination arises from every
element being an individual, having its own chemistry.