Tantalum complexes fluoro

S. Brownstein, Inoig. Chem. 1973,12, 584-589. Complex fluoro anions in solution, I. Homo-and heteropolyfluoro anions of niobium and tantalum. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

In general, Group 5 fluorides tend to adopt a preference for higher oxidation states, and as a consequence, over the past few years, there have been virtually no reported examples of low valent fluoro complexes of vanadium, niobium or tantalum. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

KCl-NaCl-K2TaF7 melts On addition of potassium heptafluotantalate to the NaCl-KCl melt a peak of tantalum reduction to metal (Rl) from fluoro-chloro complex (as will be shown below) was observed on voltammograms (Fig. 1). On the anodic part of the curve several peaks of the deposited tantalum oxidation correspond to it. The anodic peaks OJ andOJ" should be referred to tantalum dissolution respectively in the form of Ta(lV) and Ta(V) chloro complexes, formed in the absence of excess fluoride ions. Recording of the cyclic voltammetric curve with a potential reverse at more positive potential -0.48 or -0.6 V (Fig.l), when the amount of tantalum deposited at the electrode is small and there is not enough time for diffusion of a considerable quantity of the liberated fluorine into the melt bulk, shows no such peaks, which corroborates our assumption. Peaks Oi and OJ were caused by tantalum disso-... 

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