Crystal structure of CaSiF(6)·2H(2)O(mP2) and reevaluation of the Si(IV)-F bond-valence parameter R(0).

The structure of a second polymorph of CaSiF(6)·2H(2)O [calcium hexafluorido-silicate dihydrate; space group P2/c (No. 13), Pearson symbol mP2] was elucidated by single-crystal X-ray diffraction. It arose as an unexpected product when soda-lime glass was attacked by HF. Its crystal structure consists of infinite (∞) (2)[Ca(H(2)O)(2/1)(SiF(6))(4/4)] layers oriented parallel to the bc-crystallographic plane, a unique motif among structurally characterized hydrated hexa-fluorido-silicates. The crystal structure also exhibits inter- and intra-layer hydrogen bonds, with the inter-layer O-H⋯O hydrogen bonds involving a disordered hydrogen atom. The large deviation between the calculated bond-valence sum for Si and the expected value prompted a redetermination of the empirical Si(IV)-F bond-valence parameter R (0). Based on a data set of 42 high-quality crystal structures containing 49 independent Si(IV) coordination environments, a revised value of 1.534†à was derived for R (0).