Syntheses, Structure, and Bonding of Rb₁₄(Mg_1-<i>_x</i>In<i>_x</i>)₃₀. A Nonstoichiometric Phase with an Unusual Substitution in the Anion Framework

The title compound Rb14(Mg1-xInx)30 (x = 0.79−0.88) has been obtained from high-temperature reactions of the elements in welded Ta tubes. There is no analogous binary compound without Mg. The crystal structure established by single-crystal X-ray diffraction means (space group P6̄2m (No. 189), Z = 1 and a = b = 10.1593(3) Å, c = 17.783(1) Å for x = 0.851) features two distinct types of anionic layers: isolated pentacapped trigonal prismatic In117- clusters and condensed [(MgxIn1-x)5In14]7- layers. The latter consists of analogous M11 (M = Mg/In) fragments that share prismatic edges and are interbridged by trigonal M3 units. The structure shows substantial differences from related A15Tl27 (A = Rb, Cs) in which the cation A that centers a six-membered ring of Tl11 fragments is replaced by M3. Both linear muffin-tin orbital and extended Hückel calculations are used to analyze the observed phase width and site preferences. We further utilize the results to rationalize the distortion of the M11 fragment in the condensed layer and also to correlate with electrical properties. An isomorphous phase region (RbyK1-y)14(Mg1-xInx)30 (y = 0.52, 0.66 for x = 0.79) is also formed.