New examples for the unexpected stability of the 10 pi-electron Huckel arene [Si-6](10-)

The compounds Ba4Ag2Si6, Eu4Ag2Si6, and Ca4Ag2Si6, prepared from the elemen ts at 1273 K (the components in inner corundum crucibles are enclosed in se ared quartz ampoules), are brittle semiconductors with silvery luster. They react slowly with acids liberating hydrogen. Ba4Ag2[Si-6] and Eu4Ag2[Si-6] crystallize like Ba4Li2[Si-6] (space group Fddd (No. 70); a = 8.613 Angstr om, b = 14.927 Angstrom, c = 19.639 Angstrom, and a = 8.420 Angstrom, b = 1 4.585 Angstrom, c = 17.864 Angstrom, respectively), whereas Ca4Ag2[Si-6] re presents a new structure type (space group Fmmm (No. 69); a = 8.315 Angstro m, b = 14.391 Angstrom, c = 8.646 Angstrom). The three compounds are Zintl phases with the formal charges M2+, Ag+ and [Si-6](10-). The mean bond leng ths (d) over bar(Si-Si) = 2.335-2.381 Angstrom in the 10 pi-Huckel arene [S i-6](10-) as well as (d) over bar(Ag-Si) = 2.464-2.595 Angstrom vary with t he size of the M2+ cations. The chemical bonding was analyzed in terms of t he Electron Localization Function (ELF) and compared with the bonding in re lated systems (Ce4Co2Si6).