Heptavalent ions

Feng, X., Song, C. 2001. The extraction and stripping behavior of heptavalent technetium with trialkyl phosphine oxide. Solvent Extr. Ion Exch. 19 (1) 51-60. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

It is well-known that the solid-state chemistry of inorganic oxocompounds of hexa-, penta- and heptavalent actinide ions (An) is dominated by 2-dimensional (2D) layered stmctures, due to the strong tendency of An" ... 

With the metals of the first transition series, the maximum coordination number of higher oxidation states is six, and this is so firmly fixed that in their fluoride complexes the oxidation state of the metal in question can be fixed by controlling the mol fraction of alkali metal present 26). Thus, the fluorination of a vanadium salt in the presence of a one, two, or three mol ratio of potassium ion, yields KV F6, KgWVFe, or KsV Fe. The same tendency is shown, but to a lesser degree, by metals of the second transition series, as exemplified by KRuFe and KgRuFe. For an unusual example in the third series, note that OsFe is known, OsFt is not stable, but heptavalent osmium is found as six-coordinated OsOFs (27). 

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