Comparison of the Electronic Properties of f⁷, f⁸, and f⁹ Lanthanides With Formally Isoelectronic Actinides

Relativistic effects increase substantially between lanthanides and actinides inducing reorganization of frontier orbitals and large spin–orbit coupling. Spin–orbit splitting causes mixing of excited states with ground states in actinides, and thus the ground states can only be described as admixtures of multiple Russell–Saunders terms that have the same value of J. This mixing has profound effects on the electronic structure of actinides particularly beyond curium (Z = 96). In this chapter, the structures and magnetic properties of berkelium and californium compounds are compared with the formally isoelectronic lanthanides, terbium, and dysprosium. In addition, the magnetic properties of Bk^IV are compared with other f⁷ ions. Quantum mechanical studies reveal that not only are the ground states multireference, but that the heavy actinides strongly deviate from Hund's First Rule and have uneven distribution of electron spin among different m_l microstates. Furthermore, even in 5f⁷ systems, a substantial portion of the spin density is antiparallel. Departure from classical descriptions of ground states becomes more pronounced as Z increases. These features are largely absent in isoelectronic lanthanides and even substantially diminished in actinides lighter than berkelium. In addition to the demonstration of alterations in electronic structure of middle-to-late actinides, this chapter also reviews the structural chemistry of several families of lanthanide and actinide compounds and complexes in an effort to demonstrate the propagation of electronic structure alteration into changes in bond lengths and gross changes in crystal structures.