Niobium chloride clusters

Overview of the Chemistry of Niobium Chloride and Niobium Oxide Cluster Compounds 83... 

In contrast to chloride compounds, niobium oxides have a VEC of 14 electrons, due to an overall anti-bonding character of the a2u state, caused by a stronger Nb-O anti-bonding contribution. In some cases, the VEC cannot be determined unambiguously due to the uncertainty in the electron distribution between the clusters and additional niobium atoms present in the majority of the structures. The 14-electron compounds exhibit semiconducting properties and weak temperature-independent paramagnetism. Unlike niobium chlorides, the oxides do not exhibit a correlation between the electronic configuration and intra-cluster bond distances. 

The number of valence electrons in niobium oxychloride clusters decreases as the number of oxide hgands increases (Table 6.4). The compound Cs2LuNbi Cli70 has the VEC of 16 as found in most chloride clusters, suggesting that the presence of one oxide ligand per cluster does not tip the balance between the Nb-Nb bonding and Nb-L anti-bonding contributions to the a2u state. The VEC of most clusters... 

Oxidized, metastable clusters are obtained for both families via solution processes near room temperature and, in parallel, these contain 14-e niobium and tantalum members, but 12-e (and 13-e) zirconium chloride clusters. Rare-earth-metal clusters don t provide such persuasive evidence small main-group Z examples are rather scarce but similar (12-14-e for R7X-12Z), while iodides centered by large metals cover a sizable range (16-20), as was rationalized earlier (see 5). 

The electrochemical behavior of Nb(V) in oxide free basic aluminum chloride -methyl-3-ethylimidazolium chloride - is reported to involve four reduction steps, but reduction to the free metal could not be achieved [1, 2]. Niobium and tantalum chloride cluster cores of the type [M6C1i2] (M=Nb or Ta) in this same ionic liquid have been studied in detail by electrochemical and spectroscopic methods and their behavior compared with the behavior of these systems in an MeCN electrolyte. Four reversible redox states are observed in the span z = 2-5 [3, 4]. 

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

Fig. 6.9 Differentiation of niobium atoms and outer chloride ligands in the cluster unit of Ti2Nb6Cli404 and [Tl5(Ti2Cl9)][(NbeCI,204)3(Ti3Cl4)2]. The numbers in the boxes are electrostatic charges calculated by the DFT method.

Clusters of six molybdenum, niobium, or tantalum atoms have been known for muny years, predating the work with rhenium. There two types In the first, an octahedron of six metal atoms is coordinated by eight chloride ligands, one on each face of the octahedron (Fig. 16.64a). This is found in molybdenum dichloride, MoftCI,2. better formulated as [MoftCIg]CI4. Each Mo(II) atom can use its four electrons to form four bonds with adjacent molybdenum atoms and can receive dative bonds from the four chloride ligands 164... 

Metal cluster complexes were proposed to introduce mainly transition metals between the clay layers. Niobium, tantalum and molybdenum chlorides were exchanged on the clay surface and oxidized at 513 K under vacuum to transform... 

Mild oxidant agents often permit oxidations with retention of the cluster structures. One example of this kind of simple chemical electron-transfer reactions are the oxidations of niobium and tantalum clusters [M6Xi2] to the species with charges 3+ and 4+. The reaction with iron(III) chloride and cerium (IV) salts leads directly to the cations [M6Xi2] . ... 

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