Actinides: Inorganic & Coordination ChemistryBased in part on the article Actinides: Inorganic & Coordination Chemistry by Grigorii L. Soloveichik which appeared in the Encyclopedia of Inorganic Chemistry, First Edition .

Abstract

The Inorganic and Coordination Chemistry of the 5f elements (Actinides) is intermediate between that of the transition metals and the 4f elements (Lanthanides). The first four members of the actinide series, Ac, Th, Pa, and U, are all naturally occurring but have unstable nuclei, making them radioactive. The other elements in the actinide series (transuranium elements or transuranics) are also radioactive and have been made by man through a variety of nuclear transformation processes, for example, neutron irradiation, atom bombardment, and radioactive decay. The actinide series may be broken into two separate groups, the light or early actinides (Ac–Am) and the heavy or late actinides (Cm–Lr). The chemical behaviors of these two groups are distinct. The light actinides are capable of being stabilized in a variety of oxidation states, for example, II to VII, and have been found to exhibit a significant degree of covalency in binding ligands. The heavy actinides are more reminiscent of the lanthanides in that the most persistent oxidation state is the trivalent and the bonding predominately occurs through ionic interactions. This difference has led to a substantial number of studies into the role of the 5f orbitals in structure and bonding of the actinides. One of the ions that has received a great deal of attention is the ubiquitous actinyl ion, AnO22+/+. This ion is the stable form for all aqueous complexes of the pentavalent and hexavalent light actinides. The oxo ligands are always found in a trans geometry and are strongly bound (triple bond) through a combination of σ‐ and π‐bonds with the 5f and 6d orbitals of the actinide metal center. While these strong covalent interactions exist with the axial oxo ligands, the bonding within the equatorial plane is primarily based on ionic interactions. The ability of the light actinides to access multiple oxidation states leads to rich and, sometimes, complex electrochemistry. For example, under acidic conditions the reduction potentials for PuO 2 2+ , PuO 2 + , Pu 4+ , and Pu 3+ are all close to 1 V. The physical manifestation of these close reduction potentials is an aqueous solution in which the plutonium can exist in four different oxidation states simultaneously. By careful control of the conditions, oxidation state pure solutions can be obtained for quantitative and qualitative studies. The binding affinities of the actinide ions to ligands follow the general trend An 4+ > An 3+ ∼ AnO 2 2+ ≫ AnO 2 + . Since the actinides are strong acids, the ions preferentially bond strong bases, for example, halides, oxygen donors, and so on. Owing to the greater degree of covalency in actinide complexes compared to the lanthanides, complexes with ligands having nitrogen, phosphorus, sulfur, and selenium donors are also found.