Actinides dioxo ions

The known oxidation states of the actinides are given in Table 20-3. With the exception of Th and Pa, the common oxidation state, and for trans-americium elements the dominant oxidation state, is +3, and the behavior is similar to the +3 lanthanides. Thorium and the other elements in the +4 state show resemblances to HfIv or Ceiv, whereas Pa and the elements in the +5 state show some resemblances to Tav. Exceptions to the latter statement are the dioxo ions MOJ for U, Np, Pu, and Am that are related to the M02+ ions in the +6 state. Redox potentials are given in Table 20-4. 

Pentavalent actinides can also exhibit a dioxo ion, (An02), but no structures of inorganic phosphate or arsenate compounds with this moiety have yet been reported, although this is an area of ongoing research [100]. 

The actinide ions which are of importance in any discussion on their role in the biosphere are the trivalent (Pu3+, Am3+, Cm3+) tetravalent (Np4+, Pu4+) and dioxo species (U02+, NpOf, Pu02+, AmOf). 

Unlike the lanthanides, the actinides U, Np, Pu, and Am have a tendency to form linear actinyl dioxo cations with formula MeO and/or Me02. All these ions are paramagnetic except UO and they all have a non-spherical distribution of their unpaired electronic spins. Hence their electronic relaxation rates are expected to be very fast and their relaxivities, quite low. However, two ions, namely NpO and PuOl", stand out because of their unusual relaxation properties. This chapter will be essentially devoted to these ions that are both 5/. Some comments will be included later about UOi (5/°) and NpOi (5/ ). One should note here that there is some confusion in the literature about the nomenclature of the actinyl cations. The yl ending of plutonyl is often used indiscriminately for PuO and PuOl and the name neptunyl is applied to both NpO and NpOi. For instance, SciFinder Scholar" makes no difference between yl compounds in different oxidation states. Here, the names neptunyl and plutonyl designate two ions of the same 5f electronic structure but of different electric charge and... 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

Actinide complexes rarely follow the conventional rules, for example, 18-electron rule, typically found in inorganic and coordination chemistry of the transition metals. A prime example of the difference in actinide chemistry is the pervasiveness of the linear dioxo rmit, which is immatched in transition metal chemistry. For the actinides, the trans dioxo structure is maintained through different metal ions, oxidation states, and valence electron cormts, for example,... 

All neutral base adducts of hexavalent actinides are those of the dioxo, or actinyl ions (An02 ). Although at least one report has appeared which indicates the stability of base adducts of UFg. However, subsequent researchers have demonstrated that reduction occurs in these systems." ... 

It is frequently argued that a specific property of 5f g oup M(VI) nd M(V) is the formation of linear dioxo complexes, M022 and M02. Their most striking property is the coexistence of oxo and aquo ligands, which is exceedingly rare in the rest of the elements. There are characteristic differences found frequently between the oxo ions of the V- and VI-valent actinides, uranium through americium, and those of the d elements in the same valence states. 

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