New germanium-rich compounds SrCo_5-xGe₉ (x = 0.39 and 0.28) with optimized Co - Ge bonding

New germanium-rich compounds SrCo_5-xGe₉ (x = 0.39 and 0.28) were synthesized by melting mixtures of the three elements by induction heating in niobium crucibles in a water-cooled sample chamber. Both compounds were investigated by X-ray powder and single-crystals diffraction methods; the structures have been refined on the basis of single-crystal data. The compounds crystallize with a new structure type: Pnma, Z = 4, a = 13.932(4), b = 3.957(1), c = 16.838(7) Å, wR₂ = 0.064, 1099 F² values, 93 variables for x = 0.39 and a = 13.951(3), b = 3.964(1), c = 16.869(5) A, wR ₂ = 0.046, 1543 F² values, 93 variables for x = 0.28. The complex crystal structure is built up from slabs of Ge₄Co₂ octahedra, which consist of five columns of face-sharing octahdra along the b direction. The columns are connected in the ac plane by sharing vertices. These slabs are connected by {Sr@Co₅Ge₁₅} polyhedra, which share pentagonal faces of germanium atoms along b. The columns are further condensed by common edges along the a direction. Centering strontium atoms form slightly puckered layers in the ah plane. Along the c direction, the layers of strontium-centered polyhedra are interconnected by the slabs of Ge ₄Co₂ octahedra. The Co5 site of the structure shows defects (39 % and 28 %) leading to the refined composition SrCo _5-xGe₉ for the investigated crystals with x = 0.39 and 0.28. The catalytic properties of the title compound with respect to the selective hydro-genation of α,β-unsaturated aldehydes are discussed.