Localized and Delocalized Chemical Bonding in the Compounds CaNiGe₂, SrNiGe₂, and SrNiSn₂

The new compounds CaNiGe2, SrNiGe2 and SrNiSn2 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.