Actinyl

In aqueous solutions hydrolysis of the actinyl ions is important and such solutions are distinctly acidic. The reactions are complicated but, at least in the case of U02 +, it appears that loss of H+ from coordinated H2O is followed by polymerization involving -OH- bridges and yielding species such as [(U02)(0H)]+, [(U02)2(OH)2] + and [(U02)3(0H)5]+. 

Other reasons for investigating plutonium photochemistry in the mid-seventies included the widely known uranyl photochemistry and the similarities of the actinyl species, the exciting possibilities of isotope separation or enrichment, the potential for chemical separation or interference in separation processes for nuclear fuel reprocessing, the possible photoredox effects on plutonium in the environment, and the desire to expand the fundamental knowledge of plutonium chemistry. 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

Dechnicke K, Shihada A-F (1976) Structural and Bonding Aspects in Phosphorus Chemistry-Inorganic Derivates of Oxohalogeno Phosphoric Acids. 28 51-82 Denning RG (1992) Electronic Structure and Bonding in Actinyl Ions. 79 215-276 Dhubhghaill OMN, Sadler PJ (1991) The Structure and Reactivity of Arsenic Compounds. 

Rate Constants and Activation Parameters for Water Exchange on Actinides or Actinyl Aqua Ions... 

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... 

As mentioned in the Introduction, the actinyl ions are not stable under all chemical conditions. Plutonium can coexist in solution in several oxidation states, the stability of which often depends strongly on acidity (26). As a result, great care must be taken to obtain pure solutions of PuOl(27). On the other hand, the neptunyl ion NpO is the most stable form of neptunium in aqueous solution. It is noteworthy that the exchange between the oxygen atoms of PuO and H20 is very slow (ti/2 > 10 h) (25), whereas it is quite fast (h/2 2.2 s) in the case of NpO. ... 

Two other actinyl ions, UO (5/°) and NpO (5/ ), are worth mentioning here. Although the uranyl ion contains no unpaired/electrons, its complexes may display a weak temperature-independent paramagnetism because of... 

Relaxivity has also been applied to attempts at better understanding an unusual behavior of the dioxo actinyls. In 1961 itself, Sullivan et al. (55) showed that the relaxivity of solutions of NpO was increased by the... 

The aqueous chemistry of the +3 and +4 actinide ions is complicated by their tendency to hydrolyze and polymerize. Higher oxidation states are represented by stable actinyl ions (e.g., MO, MO2, and MO ). 

The actinide ions in 5+ and 6+ oxidation states are prone to severe hydrolysis as compared to lower oxidation states in view of their high ionic potentials. Consequently, these oxidation states exist as the actinyl ions MOt and MO + even under acidic conditions, which can further hydrolyze under high pH conditions. The oxygen atoms of these ions do not possess any basic property and thus do not interact with protons. The tetravalent ions do not exist as the oxy-cations and can be readily hydrolyzed at low to moderate pH solutions. The degree of hydrolysis for actinide ions decreases in the order M4 > MOT > M3 > MOt, which is similar to their complex formation properties (4). In general, the hydrolysis of the actinides ions can be represented as follows ... 

P-Diketonates. Very strong actinide complexes with p-diketones [An(acac)4 and An02(acac)2] are used in solvent extraction and separation of actinides. They are prepared by direct interaction of the metal or actinyl halide with the appropriate p-diketone in the presence of a base. Only fluorinated An(IV) diketonates produce adducts with Lewis bases, whereas common An02(acac)2 (An = Np, Pu) are stabilized by adduct formation. Fluorinated U02(hfa)2 is a very strong Lewis acid and its adducts with H20 and ROH can be sublimed without decomposition [282],... 

Abb. 15. Relative Energieniveaus der Actinyl-MolekOlorbitale mit Elektroneniiber-gangen (nach 105a>)... 

Eine der hervorstechendsten Eigenschaften der Chemie der Actiniden ist die Stabilitat des Actinyl -Ions MeO (w = 1 fiir Me(V) und n — 2 fiir Me(VI)). Eine Ausnahme davon bildet nur Protactinium, von dem kein. .yT -Ion Pa02 bekannt ist. Bei Kristallstrukturuntersuchungen... 

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