Tantalum halide clusters

A variety of polynuclear cations other than the Alj 3 Keggin ion have been used to pillar clays. Among them, we cite the zirconium tetramer [30,40,41,50,51], chromia polymers [52], bismuth [53], silicon [54], and niobium and tantalum halide clusters [55]. Of these, the most thoroughly studied have been zirconium polymers. 

Within the (M6Xi2) + (X = Cl, Br q = 2-4) cluster halides, the only series that cover all three oxidation states are the tantalum halide hydrates and the haloanions [(MsXi Xg] -. The isolation of [(Ta6Cli2)Cl2(PR3)4](PF6) (n = 1, 2) led to the first complete series of group VA cluster stabilized by an organic ligand. 

The second class of hexanuclear clusters also contains an octahedron of metal atoms, but they are coordinated by twelve halide ligands along the edges (Fig. 16.64b). Niobium and tantalum form clusters of this type. Here the bonding situation is somewhat more complicated The metal atoms are surrounded by a very distorted square prism of (bur metal and four halogen atoms. Furthermore, these compounds are electron deficient in the same sense as the boranes—there are fewer pairs of electrons than orbitals to receive them and so fractional bond orders of are obtained. 

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

Formation of Heavy Transition Metal Group V Metal-Metal Bonds 9.2.8.2. In Niobium and Tantalum Halides Without Hexanuclear Cluster Units... 

In Niobium and Tantalum Halides Without Hexanuclear Cluster Units... 

The scientific community did not realize the importance of cluster compounds for nearly 40 years after their discovery. In the early 1900s tantalum halides of the general formula [Ta6Xi2] + were being prepared. In 1907 the chloride was incorrectly reported as TaCb. In 1910 the... 

Ligand Exchange in Metal Halide Clusters. Rhenium, molybdenum, tantalum and niobium halides are high-valence metal clusters that formally behave as Lewis acids towards halide ions forming anionic complexes that retain the structure of the original cluster. 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

The octahedral metal clusters that have long been familiar features of the lower halide chemistry of niobium, tantalum, molybdenum, and tungsten represent a category of cluster different from those so far considered in that their metal-metal bonding is best treated as involving four AO s on each metal 49, 133,144,165,178). 

The lower halides of niobium and tantalum consist of tightly bound clusters of metal atoms, with metal-metal distances close to those found in the metal. They contain ions with average oxidation numbers between +III and +1 (Table 43). Their size depends on the valence electron concentrations (VEC) that are available on the metal atoms for M—M bonding, and on the halide-metal ratio.644 Several reviews have been devoted to the clusters of early transition metals.3,643... 

In these clusters tantalum atoms are bound to other tantalum atoms and are also edge bridged via halide. As our deposit was completely amorphous without any XRD peak we concluded that it did not consist of crystalline tantalum but rather of such clusters. We varied the electrode potential for deposition and tried deposition with very low constant current densities, but in no case was crystalline tantalum obtained. Thus, the electrochemical window of our liquid was surely wide enough, but for some reason the electrodeposition stopped before Ta(0) was obtained. When we studied the literature dealing with metal clusters we found that the cluster chemistry with fluoride seems to be less comprehensive. Consequently... 

The most convenient route to M6Yi2 2+ clusters of niobium and tantalum is the conproportionation of the pentahalides NbY5 and TaY5 (Y = Cl, Br) with excess of the metal in molten alkali halide ... 

---