Tetrahedral coordination for Zn in hexacyanometallates: Structures of Zn₃A₂[M(CN)₆]₂·xH₂O with A=K, Rb, Cs and M=Ru, Os

Zinc hexacyanoruthenate (II) and hexacyanoosmate (II) were prepared and studied from X-ray diffraction (XRD), infrared (IR), and thermogravimetric (TG) data. These compounds were found to be isomorphous with the iron analogues, crystallizing with a rhombohedral unit cell (R-3c space group), where the zinc atom has tetrahedral coordination to N ends of CN groups. For Cs, compounds with formula unit ZnCs₂[M(CN)₆] and a cubic unit cell (Fm-3m) were also obtained. The crystal structures for the eight compositions were refined from the corresponding X-ray powder diffraction patterns using the Rietveld method. Related to the tetrahedral coordination for the Zn atom, the rhombohedral phase has a porous framework with ellipsoidal cavities of about 12.5×9×8 Å, communicated by elliptical windows of ∼5 Å. Within these cavities the exchangeable alkali metal ions are found. The filling of the cavity volume is completed with water molecules. IR spectrum senses certain charge delocalization from the inner metal, through the π-back donation mechanism. For Os compounds this effect is particularly pronounced, related to a more diffuse d orbitals for this metal.