Localized and delocalized chemical bonding in the compounds CaNiGe ₂, SrNiGe₂, and SrNiSn₂

The new compounds CaNiGe₂, SrNiGe₂ and SrNiSn ₂ have been synthesized from the elements by arc melting techniques with subsequent annealing of the sample at 1270 K, and their structures have been determined by single-crystal X-ray diffraction methods. They crystallize in the CeNiSi₂ structure (space group Cmcm). For CaNiGe₂: a = 4.2213(7) Å, b = 17.375(4) Å, c = 4.0514(7) Å, R₁ = 0.033 (all data); for SrNiGe₂: a = 4.429(1) Å, b = 17.420(4) Å, c = 4.200(1) Å, R₁ = 0.041 (all data); and for SrNiSn₂: a = 4.5924(7) Å, b = 18.710(3) Å, c = 4.5228(6) Å, R₁ = 0.021 (all data). The main structural motifs are two-dimensionally condensed Ni-centered Ge₅ or Sn₅ square pyramids. The crystal chemistry and chemical bonding are discussed. Analyses of the electronic structures of CaNiGe₂, SrNiGe₂, and SrNiSn₂, with the help of the electron localization function (ELF), indicate the coexistence of localized covalent and delocalized bonding between the metal atoms involved.