Accurate synchrotron radiation Δρ maps for K2SiF6 and K2PdCl6

Difference electron densities for the structurally isomorphic title compounds have been measured using 0.9 and 0.7 A synchrotron radiation as well as Mo Kα (λ = 0.71069 A) radiation. The merits of using synchrotron radiation for such accurate electron density studies are confirmed. The noise level in the 0.9 A K2SiF6 Δρ maps is low, and justifies confidence in physical properties predicted from the one-electron density. Similar Δρ features are present in both compounds. The redistribution of electron density indicated by the Δρ maps, which mainly reflects the effect of exchange interactions, does not support simple predictions from ionic and orbital models for the bonding in these structures. (I): Dipotassium hexafluorosilicate, K2SiF6, Fm anti 3m, cubic, Mr = 220.27, a = 8.1419 (3) A, V = 539.73 (3) A3, Z = 4, Dx = 2.711 Mg m-3, μ(0.9 A) = 3.919, μ(0.7 A) = 1.909, μ(Mo Kα) = 2.007 mm-1, F(000) = 424, T = 293 K, R = 0.021, wR = 0.017, S = 4.458 for 155 unique reflections measured with 0.9 A data. (II): Dipotassium hexachloropalladate, K2PdCl6, Mr = 397.3, Fm anti 3m, cubic, a = 9.7192 (7) A, V = 918.1 (2) A3, Z = 4, Dx = 2.875 Mg m-3, μ = 8.8571 mm-1, F(000) = 744, T = 293 K, R = 0.023, wR = 0.018, S = 5.041 for 241 unique reflections at 0.9 A. (orig.)