Inorganic Niobium and Tantalum Octahedral Cluster Halide Compounds with Three-Dimensional Frameworks: A Review on Their Crystallographic and Electronic Structures

Abstract

This review summarizes the development of the rich crystal and bonding chemistry of facecapped and edge-bridged inorganic niobium and tantalum octahedral cluster halide compounds, with a particular emphasis on those showing three-dimensional cluster frameworks. Discussion is made on varied structures and bonding which are intimately linked to the valence electron concentration, i.e., the number of electrons that held the octahedral metal cluster architecture. Exploration of the literature indicates that apart from Nb6I11 and derivatives, which show electron-deficient face-capped M6X i 8X a 6 units, compounds containing edge-bridged M6X i 12X a 6 motifs are the most largely encountered. Closed-shell compounds with a valence electron concentration of 16 are predominant, although a few 15-electron open-shell magnetic compounds or even 14-electron closed-shell species have also been reported. Particularly interesting from a structural point of view is the fashion in which these facecapped and edge-bridged clusters "pack" in crystals. The astonishing diversity of structural types, which are observed, is mainly due to the flexibility of the halogen ligands to coordinate in various manners to metal atoms. However, a rigorous structural analysis of these compounds reveals no close relationship between the valence electron concentration and the variability of the intercluster connections and/or the nature of the counter-ions. Indeed, the main bonding features of these compounds can be understood from the delocalized bonding picture of isolated 'molecular-like' M6X i 8X a 6 or M6X i 12X a 6 clusters. Keywords Crystal structure  Electronic effect  Electronic structure  Halide  Inorganic compound  Interatomic distance  Intercluster distance  Ligand  Matrix effect  Metal-metal bond  Niobium  Octahedral cluster  Structure type  Tantalum  Three-dimensional framework  Valence electron concentration Contents 5 Cluster Frameworks of Structural Formula [M6X i 12X a-a 6/2] 5.1 Cluster Compounds with Linear M-X a-aM Bridges 5.2 Cluster Compounds with Bent M-X a-aM Bridges in Cubic Structure 5.3 Cluster Compounds with Bent M-X a-aM Bridges in Trigonal Structure 5.4 Cluster Compounds with Both Linear and Bent M-X a-aM Bridges 6 Interatomic Distances in Inorganic Nb6 and Ta6 Cluster Halide Compounds with Threedimensional Frameworks 6.1 Cluster Units Based on Face-capped M6X i 8X a 6 Building Blocks 6.2 Cluster Units Based on Edge-bridged M6X i 12X a 6 Building Blocks 7 Structural Relationships Between Crystal Structures Based on Hexagonal, Cubic and Trigonal Symmetry 8 Electronic Structure of Niobium and Tantalum Octahedral Cluster Halide Compounds 8.1 Variable VEC of Nb6I11 and Derivatives 8.2 Electronic Structure of Edge-bridged M6X i 12X a 6 Clusters 9 Summary References

An Ab Initio Study of Clusters as Building Blocks for Crystals: From Prussian Blue Analogs to Hybrid Perovskites

“…In the following, we present the results of DFT calculations on atomic and electronic structures of cluster‐based compounds containing the cluster (Nb 6 Cl 12 ) 2+ , namely Nb 6 Cl 14 , [ 1,2 ] K 4 Nb 6 Cl 18 , [ 2,3 ] the Prussian blue analog (Me 4 N) 2 [MnNb 6 Cl 12 (CN) 6 ], [ 4 ] and hypothetical hybrid organic–inorganic perovskites. We first focus on the first three compounds, whose synthesis was reported in the literature.…”

“…The electronic structure of the (M 6 Cl 12 ) 2+ cluster remains basically unchanged in the different chemical environments and crystal structures, strengthening the idea that it could be used as a building block for novel materials. In the following, we first present the details of the employed computational methods, then we present the results for the existing compounds, namely Nb 6 Cl 14 , [ 1,2 ] K 4 Nb 6 Cl 18 , [ 2,3 ] and the Prussian blue analog (Me 4 N) 2 [MnNb 6 Cl 12 (CN) 6 ], [ 4 ] and for the hypothetical class of hybrid perovskites. Finally, a conclusion closes the article.…”

[Nb6Cl14(pyrazine)4], a Versatile Precursor for Ligand-Supported Hexanuclear Niobium Cluster Compounds: Synthesis, Characterization, Follow-Up Reactions, and Intermolecular Interactions

“…Hexanuclear niobium chloride cluster compounds are known to exist in three stable oxidation states with charges of n = 2, 3, or 4 for the cluster core [Nb 6 Cl 12 ] n + . Each oxidation state of the cluster core is associated with a certain number of electrons localized in basically metal–metal bonding states, called “cluster-based electrons” (CBEs), which are 16, 15, or 14 for n = 2, 3, or 4, respectively. ,, Consequently, the charge of the complete cluster unit depends more on the number of bonded neutral or anionic ligands. The oxidation state of the cluster core is correlated with the Nb–Nb distances, which differ by approximately 0.05 Å.…”

“…Niobium cluster compounds, which contain ligand-surrounded octahedral Nb 6 metal atom cores, have been investigated for a long time. − Because of a large variety of possible compositions, structures, and properties, the synthesis of new compounds and their physicochemical characterization have been and still are being intensively investigated. − Unfortunately, for the synthesis of new Nb 6 cluster compounds, a number of obstacles have to be conquered. One originates in the property of Nb 6 cluster compounds to be oxidized easily with the need to avoid contact with air, the low solubility of the so far used starting materials in common solvents resulting in long reaction times or high reaction temperatures, and elaborate workup procedures. − A further difficulty is the growth of (single) crystalline samples, because often a large salt and side product concentration exists, aggravating the crystallization of the target material.…”

Strong Cluster-Supported Brønsted Acids: Hexanuclear Niobium Cluster Compounds with Protonated Crown Ether Cations: (Crown-H)2[Nb6Cl12iX6a] (X = Cl or Br) and the Intermediate [Nb6Cl16(H2O)2]·4 dioxane

“…They all can be described by the general formula [ M 6 X i 12 Y a 6 ] n with M = electron-poor metal, X i = ligand on inner , μ 2 (edge) bridging site, Y a = outer , exo-bonded ligand, and n = +2 to −4 . Such clusters with edge-bridged metal atom octahedra have been found to exist in three oxidation states, which are characterized by 16, 15, or 14 electrons in the highest occupied molecular orbitals, which are basically metal-centered cluster-based electrons (CBEs). − …”