Metal arrays in structural units based on anion-centered metal tetrahedra

Structural units based on anion-centered metal tetrahedra (XA(4); X = O, N; A = metal) are described as eutactic metal fragments with anions in tetrah edral interstices. In this respect these units may be subdivided into fluor ite derivatives and units based on stellae quadrangulae (tetrahedral stars) . To describe the geometry of the metal arrays a set of tetrahedrally packe d metal radii, r(tp), is derived for A = Cu, Pb, Bi, and some rare-earth me tals from the systematic analysis of the A...A distances within (XA(4)) tet rahedra. Analysis of these radii and of the structural geometry of the unit s shows that the insertion of anions into tetrahedral interstices of the me tal fragment causes its expansion and distortions by nonbonded anion-anion repulsions. The main effect is owing to the linkage of (XA(4)) tetrahedra v ia edges, which leads to compression of the shared A...A edges and stretchi ng of the unshared edges. The geometry of this effect is described by some empirical expressions. It is suggested that the eutactic arrangement of met al atoms in structural units based on anion-centered metal tetrahedra is ca used by the closed-shell metal-metal bonding interactions.