Abstract
The compounds MNi21B20 (M = In, Sn) have been synthesized and their cubic crystal structure determined (space group Pm3[combining macron]m, lattice parameters a = 7.1730(1) Å and a = 7.1834(1) Å, 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 [Ni1@Ni38], [M@Ni312], and [Ni26@B20@Ni324], respectively. The [Ni6@B20] motif inside the rhombicuboctahedral voids features an empty [Ni6] octahedron surrounded by a [B20] 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@Ni6] trigonal prisms as basic building blocks. The shortest B-B contacts display locally nested 3-center B-B-Ni bonding inside each trigonal prism. This clearly rules out the notion of [Ni6@B20] clusters and leads to the arrangement of 20 face-condensed [B@Ni23Ni33] trigonal prisms resulting in a triple-shell like situation Ni26@B20@Ni324(reo-e), where the shells display comparable intra- and inter-shell bonding. Both compounds are Pauli paramagnets displaying metallic conductivity.
Highlights
Metal-borides feature a perplexing variety of crystal structures
We report on the synthesis, crystal structure, chemical bonding and physical properties of MNi21B20 (M = In, Sn) and place these compounds into context with structurally related metal-rich borides Zn2Ni21B20 and Cr23C6 derivatives
New borides MNi21B20 (M = In, Sn) were synthesized and their crystal structures were solved from synchrotron powder X-ray diffraction data
Summary
Metal-borides feature a perplexing variety of crystal structures. Numerous and systematic studies suggest that saturation of the valence requirements of the electron-deficient boron constituent is the actual driving force of this complexity.[1,2,3] In borides, the complexity of boron-based structural units often depends on metal-to-boron ratios, resulting in formation of one-, two-, and three-dimensional arrangements of covalently bonded boron atoms.[1,3,4,5,6,7,8,9,10] Boron-rich materials mostly exhibit a three-dimensional boron partial structure formed by interlinked boron clusters.[3,11,12] Two-dimensional boron networks in turn can be often observed at intermediate metal-to-boron ratios between 3/2 and 2,3,13,14 whereas, with decreasing boron content, separated chains or rings and, further, isolated boron atoms prevail.[15,16,17,18,19]studying the phase diagram at intermediate compositions enables one to follow the structural evolution of and bonding interactions between boron aggregates and metal framework observed in complex intermetallic compounds. The 2-center delocalization indices (DIs) between QTAIM atoms, a position-space variant of an effective covalent bond order, were calculated according to ref.
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