Exploratory synthesis in the solid state. Endless wonders.

This article gives an overview of recent developments in three areas of solid state chemistry: (1) The discovery that centered and originally-adventitious interstitial elements Z are essential for the stability of M6X12-type cluster halides of group 3 and 4 metals has led to a large amount of new chemistry through tuning structures and compositions of AnM6(Z)X12Xn phases with the variables Z, x, and n. The corresponding metal-rich group 3 tellurides exhibit novel and more extensive metal aggregation, reflecting a decreased number of anions and valence electrons. (2) Many intrinsically metallic T5M3 phases with a Mn5Si3-type structure are formed by early transition metals T with main-group elements M. Each characteristically reacts with diverse elements (up to 15-20 each) to form stuffed interstitial versions T5M3Z of the same structure. The ranges of Z and some properties are described. Related reactions of hydrogen (often as an impurity) in Mn5Si3-, beta-Yb5Sb3-, and Cr5B3-type systems are extensive. Substantially all previous reports of beta-Yb5Sb3- and Cr5B3-type phases for divalent metals with pnictogen (As-Bi) and tetrel (Si-Pb) elements, respectively, have been for the hydrides, and about two-thirds do not exist without that hydrogen (or fluorine). (3) The developing chemistry of anionic polymetal cluster compounds of the main-group elements with alkali-metal cations is outlined, particularly for the triel elements In and Tl. These clusters lie to the left of what has been called the Zintl boundary, many are new hypoelectronic polyhedra, some may be centered by the same or another neighboring element, and so far all have been isolated only as neat solid state compounds in which specificity of cation-anion interactions seems important. Extended networks are also encountered.