Actinyl ions formation

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 ... 

In both families of elements, the eations are typical hard acids. Their outermost electronic configuration resembles that of the closed-shell systems (e.g., inert gases, alkali, and alkaline-earth cations) as the inner f orbitals are largely or completely unavailable for bond formation. There is evidence for covalent bonding in actinide chemistry in the formation of the actinyl ions, AnOj and AnO in which both f and d orbitals participate in the An-O bonds, but the extent and even the existence of covalency in the bonding of simple An" species is a subject of controversy. The presence of a slight amount of covalency in lanthanide bonds has been attributed to involvement of the lanthanide 6s orbitals rather than of the 4f orbitals (Lewis et al. 1962). Differences in interactions with soft donor atoms have been the basis of a... 

Table 2 summarizes orbital populations for both the actinide atoms and the donor oxygen atoms. The electron configurations for the ideal perfect ions which are defined for formal charges in these actinyl nitrates are [Rn] 5f6d for U(VI), [Rn] 5f 6d for Np(VI) and [Rn] 5f-6d for Pu(VI). Any variation in the orbital populations from those of the formal configurations means that to some extent there is some covalent bond formation. A significant changes in the orbital population is an increase in the number of An 5f, 6d electrons. 

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