ACCURATE SYNCHROTRON-RADIATION DELTA-RHO MAPS FOR K2SIF6 AND K2PDCL6

Difference electron densities for the structurally isomorphic tide com pounds have been measured using 0.9 and 0.7 angstrom synchrotron radia tion as well as Mo Kalpha (lambda = 0.71069 angstrom) radiation. The m erits of using synchrotron radiation for such accurate electron densit y studies are confirmed. The noise level in the 0.9 angstrom K2SiF6 DE LTArho maps is low, and justifies confidence in physical properties pr edicted from the one-electron density. Similar DELTArho features are p resent in both compounds. The redistribution of electron density indic ated by the DELTArho maps, which mainly reflects the effect of exchang e interactions, does not support simple predictions from ionic and orb ital models for the bonding in these structures. (1): Dipotassium hexa fluorosilicate, K2SiF6, Fm3mBAR, cubic, M(r) = 220.27, a = 8.1419 (3) angstrom, V = 539.73 (3) angstrom3, Z = 4, D(x) 2.711 Mg m-3, mu(0.9 a ngstrom) = 3.919, mu(0.7 angstrom) = 1.909, mu(Mo Kalpha) = 2.007 mm-1 , F(000) = 424, T = 293 K, R = 0.021, wR = 0.017, S = 4.458 for 155 un ique reflections measured with 0.9 angstrom data. (II): Dipotassium he xachloropalladate, K2PdCl6, M(r) = 397.3, Fm3mBAR, cubic, a = 9.7192 ( 7) angstrom, V = 918.1 (2) angstrom3, Z = 4, D(x) = 2.875 Mg m-3, mu = 8.8571 mm-1, F(000) = 744, T = 293 K, R = 0.023, wR = 0.018, S = 5.04 1 for 241 unique reflections at 0.9 angstrom.