Valence compounds at the border to intermetallics: alkali and alkaline earth metal stannides and plumbides

The element Sn has the outstanding property that it appears in a metallic a nd a semi-metallic modification, which possess almost the same stability. T he associated interplay between delocalized and localized bonding governs t he structures of alkali and alkaline earth metal stannides and leads to a l arge variety of different structure types. Even though the element Pb cryst allizes in the fee lattice, which is typical for metals, directed bonds bet ween Pb atoms occur in compounds with electropositive metals. During the last decade several new structures in the phase systems of alkal i or alkaline-earth metals with tin or lend were characterized. Many exampl es show the gradual transition from valence compounds to intermetallic phas es and present new possibilities for structural and bonding motives for Sn and Pb. These structures reinforce interesting links between the various st ructure types. Structural relationships of Zintl phases containing discrete and linked polyhedra, as well as the gradual transition to typical interme tallic compounds are highlighted.