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Abstract

The compounds MNi21B20 (M = In, Sn) have been synthesized and their cubic crystal structure determined (space group Pm (3) over barm , lattice parameters a = 7.1730(1) A and a = 7.1834(1) A, respectively). The structure can be described as a hierarchical partitioning of space based on a reo-e net formed by Ni3 species with large cubical, cuboctahedral and rhombicuboctahedral voids being filled according to [Ni-1@Ni3(8)], [M@Ni3(12)], and [Ni2(6)@B-20@Ni3(24)], respectively. The [Ni-6@B-20] motif inside the rhombicuboctahedral voids features an empty [Ni-6] octahedron surrounded by a [B-20] cage recently described in E2Ni21B20 (E = Zn, Ga). Position-space bonding analysis using ELI-D and QTAIM space partitioning as well as 2- and 3-center delocalization indices gives strong support to an alternative chemical description of space partitioning based on face-condensed [B@Ni-6] trigonal prisms as basic building blocks. The shortest BB contacts display locally nested 3-center BBNi bonding inside each trigonal prism. This clearly rules out the notion of [Ni-6@B-20] clusters and leads to the arrangement of 20 face-condensed [B@Ni2(3)Ni3(3)] trigonal prisms resulting in a triple-shell like situation Ni2(6)@B-20@Ni3(24)(reo-e), where the shells display comparable intra- and inter-shell bonding. Both compounds are Pauli paramagnets displaying metallic conductivity.