SYNTHESIS AND CRYSTAL-STRUCTURE OF ALKALI-METAL DIAMIDO DIOXOSILICATES M2SIO2(NH2)2 WITH M-CORRESPONDS-TO-K, RB AND CS

SiO2 - alpha-quartz - reacts with alkali metal amides MNH2 (M = K, Rb, and Cs) in molar ratios from 1 : 2 to 1 : 10 at 450-degrees-C less-than-or-equal-to T less-than-or-equal-to 600-degrees-C and P(NH3) = 6 kbar in autoclaves to diamidodioxosilicates M[SiO2(NH2)2]. Crystals of the colourless compounds which hydrolyze rapidly were investigated by x-ray methods. Following data characterize the structure determination on the isotypic compounds: M2[SiO2(NH2)2], P2(1)/c, Z = 4 M = K a = 6.711 (1) angstrom, b = 10.075(4) angstrom, c = 7.636(2) angstrom, beta = 91.04(2)-degrees N(F0) = 568, N(Var.) = 81, R/R(w) = 0.034/0.041 M = Rb a = 6.851(1) angstrom, b = 10.370(5) angstrom, c = 7.938(1) angstrom, beta = 91.58(1)-degrees N(F0) = 996, N(Var.) = 64, R/R(w) = 0.038/0.051 M = Cs a = 7.084(1) angstrom, b = 10.748(1) angstrom, c = 8.356(1) angstrom, beta = 92.52(1)-degrees N(F0)) = 3036, N(Var.) = 64, R/R(w) = 0.039/0.053 The structures of the diamidodioxosilicates are closely related to the beta-K2SO4 type. They contain isolated [SiO2(NH2)2 ]2- ions. K+ ions and hydrogen bridge bonds N-H...O (with 2.68 angstrom less-than-or-equal-to d(N...O) less-than-or-equal-to 2.78 angstrom for the K compound) connect the tetrahedral anions.