Revision Note on Actinide Chemistry
The chemical properties of the Actinides show less uniformity across the series as compared to Lanthanides. However, the radioactivity associated with most of the actinides has hindered their study. Because the later actinides are available in such tiny amounts, little is known about their reactions. The early Actinides, particularly uranium and plutonium, are of great importance in the generation of power through nuclear fission and their chemical properties have been investigated thoroughly.
It has been observed that early actinoids do not exhibit the chemical uniformity of the lanthanides. They tend to exist in diverse oxidation states in which An (III) becomes more stable across the series.
The 14 elements from thorium (Th, Z = 90, Sf1) to lawrencium (Lr, Z = 103, 5f14) involve the completion of the 5f subshell but they do not exhibit the chemical uniformity like lanthanoids. Like lanthanoids, a common oxidation state of the actinoids is An(III), unlike the lanthanoids, the early members of the series occur in a rich variety of other oxidation states. Unlike the lanthanides, the f orbitals of the early actinoids extend into the bonding region, so the spectra of their complexes are strongly affected by ligands. The 5f and 6d orbitals are less compact as compared to 4f and 5d orbitals and the electrons present in them are more available for bonding. Thus, the outermost electron configuration of U is 5f36d17s2 with all six electrons available for bonding.
The major difference between the chemical properties of the lanthanoids and early actinoids led to controversy about the most appropriate placement of the actinides in the periodic table. The similarity of the heavy actinoids and the lanthanide is illustration like the lanthanides, the actinides have large atomic and ionic radii (the radius of an An3+ ion is typically about 5 pm larger than its Ln3+ congener) and as a result, often have high coordination numbers.
For early actinoids, the electronic spectra have contributions from ligand to metal charge transfer, 5f → 6d, and 5f → 5f transitions. Transitions between the electronic states involve only f orbitals, 5f and 6d orbitals, and ligand-to-metal charge transfer (LMCT) are all possible for the actinide ions. The f-f transitions are broader and more intense as compared to lanthanoids. This is because 5f orbitals interact more strongly with the ligands. Their molar absorption coefficients typically lie in the range 10−100 dm3 mol−1 cm. The most intense absorptions are associated with LMCT transitions. For instance, LMCT transitions result in the intense yellow colour of the uranyl ion, , in solution and its compounds. In species such as U3+ (f3) transitions such as 5f26d1 ← 5f occur at wavenumbers between 20 000 and 33.000 cm-1 (500-300 nm) giving solutions and compounds of this ion a deep orange-red colour. For Np3+ and Pu3+, with increasing effective nuclear charge, the separation of the 5f and 6d levels increases and the corresponding transitions move into the UV region of the spectrum; Np3+ solutions are violet and those of Pu3+ are light violet-blue due mainly to f − f transitions.
Thorium and Uranium
The common nuclides of thorium and uranium exhibit only low levels of radioactivity due to which their chemical properties have been extensively developed; the uranyl cation is found in complexes with many different ligand donor atoms; the organometallic compounds of the elements are dominated by pentamethylcyclopentadienyl complexes.
Neptunium, Plutonium, and Americium
Oxidation states higher than +3 become increasing less accessible between Np and Am, although all these actinoids can form AnO2*+, species in aqueous solution. The three elements Np, Pu, and Am form compounds containing similar species, although there are significant differences in the stabilities of the main oxidation states.
Neptunium dissolves in dilute acids to produce Np3+, which gets readily oxidized by air to produce Np4+. Increasingly strong oxidizing agents produce NpO2+ (Np(V)) and NpO22+ (Np(VI)). The four oxidation states of plutonium, Pu(III), Pu(IV), Pu(V), and Pu(VI), are separated from each other by less than 1 and solutions of Pu often contains a mixture of the species Pu3+, Pu4+, and PuO22+ (PuO2+ has a tendency to disproportionate to Pu4+ and PuO22+). The ion Am3+ is the most stable species in solution, reflecting the tendency for the An(III) to dominate actinide chemistry for the high atomic number elements. Under strongly oxidizing conditions AmO+2 and AmO22+ can be formed; Am(IV) undergoes disproportionation in acidic solutions.
The An(IV) oxides NpO2, PuO2, and AmO2, which are formed by heating the elements or their salts in air, all adopt the fluorite structure. Lower oxides include Np3O3, Pu2O3 and Am2O3. The trichlorides, AnCl2, can be obtained by direct reaction of the elements at 450°C and have structures analogous to that of LnCl3, with a nine-coordinate.
Tetrafluorides are known for all three actinides though only Np and Pu form tetrachloride. Both Np and Pu form hexafluorides which, like UF6, are volatile solids.
All three metals have species analogous to the uranyl ion, forming, and, which can be extracted from aqueous solution by tributyl phosphate as AnO2(NO3)2,(OP(OBu)3)2. The tetrahalides are Lewis acids and form adducts with electron-pair donors such as DMSO, as in AnCl4, (Me2SO)7, Neptunium forms a number of organo-metallic compounds that are analogues of those of uranium such as Np(Cp)4.
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