Transuranium elements oxidation state stability

Among the elements known before transuranium elements started to be synthesized in 1940, uranium has a unique characteristic, the extreme stability of the triatomic uranyl ion OUO+z. Not only are the numbers of uranium(VI) compounds larger than of U(IV), and far larger than of the two other oxidation states U(V) and U(III) known from non-metallic compounds, but until the preparation of UOFj discussed below, the only two U(VI) compounds known to contain less than two oxygen atoms per uranium atom were the octahedral molecules UFe and UQ6. 

It should be noted that some data on complexation of transuranium elements with various N-donor ligands, such as azide ion, isothiocyanate ion, amine-N-polycarboxylic and heterocyclic acids, 8-hydroxyquinoline and its derivatives, were published as early as 1950-1970 [42-77], The quantitative parameters of complexation (number and composition of complexes, stability constants, thermodynamic parameters) were determined using accessible at that time methods (spectrophotometry, potentiometry, ion exchange, solvent extraction). It was shown that transuranium elements in different oxidation states can form complexes with N-donor ligands. However, only presumptive conclusions regarding structure of complexes were drawn at that time,... 

Excluded here are those techniques relating to tracer-level work (below weighable quantity of sample measurement by radioassay only), for the concern here will be with the determination of bulk properties of these elements. Tracer-scale studies usually reveal directly only one property of the element under investigation, that is, its relative preference for one environment over another, or more simply, its phase distribution. Nevertheless, as each new transuranium element was discovered and was available only in trace quantities, a great deal of chemistry was learned by inference from tracer-scale studies, including the identity of oxidation states, approximate values of oxidation or reduction potentials, the composition and stability of complex ions, and relative volatilities. 

As early as 1923 N. Bohr suggested that there might exist a group of IS elements at the end of the Periodic Table Uiat would be analogous in their properties to the IS lanthanide ("rare earth") elemrats. This idea, combined with the increasing stability of the -t-3 oxidation state for the transuranium elem ts as the atomic number increases from Z = 93 to 96, led Seaborg to the conclusion that these new elemrats constituted a second rare earth series whose initial member was actinium. As the atomic number increases from 90, electrons are added in the Sf subshell similar to the occupation of the 4f subshell in the lanthanides, see Table 16.1. This series would be terminated with element 103 since this would correspond to the addition of 14 electrons for a completed Sf subshell. 

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