Synthesis, Crystal Structure, and Vibrational Spectroscopy of the Alkali Diselenates A 2 Se 2 O 7 ( A = Li – Cs)

The reaction of alkali carbonates and selenium acid yielded the “pyroanions” [Se 2 O 7 ] 2– containing alkali diselenates. By varying the alkali carbonates we were able to synthesize and determinate the crystal structures of the whole row from Li to Cs. Li 2 Se 2 O 7 crystallizes isotypic to Li 2 S 2 O 7 [ Pnma , Z = 4, a = 13.815(3), b = 8.452(2) c = 5.0585(10) Å]. The structure of Na 2 Se 2 O 7 [ P $\bar{1}$ , Z = 2, a = 6.9896(14), b = 6.9938(14), c = 7.0829(14) Å, α = 83.32(3), β = 64.56(3), γ = 83.18(3)°] is isotypic to Ag 2 S 2 O 7 . A 2 Se 2 O 7 ( A = K, Rb) [ A = K: C 2/ c , Z = 4, a = 12.851(3), b = 7.5677(15), c = 7.5677(15) Å, β = 93.35(3)°; A = Rb: C 2/ c, Z = 4, a = 13.118(3), b = 7.7963(16), c = 7.7811(16) Å, β = 94.03(3)°] are isotypic to K 2 S 2 O 7 . The crystal structure of Cs 2 Se 2 O 7 [ P $\bar{1}$ , Z = 10, a = 7.7271(3), b = 16.2408(8), c = 18.4427(8) Å, α = 89.685(2), β = 89.193(2), γ = 76.251(2)°] seems to be isotypic to the averaged room‐temperature modification of Cs 2 S 2 O 7 . With exception of the caesium compound all diselenate anions show an ecliptic arrangement and can be therefore classified as dichromate‐like structures. In Cs 2 Se 2 O 7 most of the [Se 2 O 7 ] 2– units have a staggered alignment. The transition between both orientations can be explained by the increase of the cations size. Additionally the vibrational spectra of A 2 Se 2 O 7 with A = Li – Cs are discussed as well as the resulting bond forces.