Tungsten halide clusters

Lowering the temperature at which cluster-forming solid-state reactions can be carried out is of some interest, particularly as a means of gaining a measure of kinetic control over the process. Several decades ago, it was discovered that the yields for forming tungsten halide cluster phases could be improved by... 

Fig. 31. The excited state efficiency 4es as a function of the reaction exothermicity AGe (data from [198, 199]) in the annihilation reactions of MoeClJj with the tungsten halide cluster acceptor WXgYj and Mo6Cl[ 4 in dichloromethane solutions. AGe values are calculated from the differences in the standard redox potentials corrected for the Coulombic interactions between reactions and product. The shape of the parabolic curve corresponds to the outer reorganization energy An = 1.2eV.

Tungsten halide cluster ions 54 Two-level tunneling 73... 

Jackson, J. A. Turro, C. Newsham, M. D. Nocera, D. G., Oxygen quenching of electronically excited hexanuclear molybdenum and tungsten halide clusters, J. Phys. Chem. 94 (1990) 4500-4507. 

Tungsten halides, halogeno and 0x0 complexes, as well as cluster compounds are obtained by reaction with halides. W can also form carbonyl, tertiary phosphine, cyano, and dinitrogen complexes. 

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). 

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. 

Near quantitative yields are achieved for the molybdenum halides, but substantially lower yields (50%) are obtained in the synthesis of the tungsten clusters, which require higher temperatures and the use of Vycor reaction vessels. A higher yield low-temperature synthesis was devised by McCarley and co-workers for W6C112 using iron as the reducing agent (14). 

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