The chemistry of an element is dictated by the energy and symmetry of its most energetic, or valence, electrons. These electrons are used to classify chemical behavior throughout the Periodic Table. A team began studying the chemistry of protactinium, an element at the bottom of the Periodic Table in the actinide series. The valence electrons in protactinium exhibit unusual symmetries and energies. They showed that the element sits at an intersection of transition metals, like iron, and heavier actinides, like plutonium. Protactinium links lighter transition metals with actinides. Why study this dichotomy? It is vital to understanding the chemical periodic system.
Since their discovery and synthesis, the chemical behavior of protactinium and other actinides has been recognized as unique in the Periodic Table. This is in part because of the electrons that govern actinide chemistry. These electrons give rise to a complex and rich chemical behavior. Studying the actinide elements and their electrons are critical. Why? It will change our fundamental understanding of chemical bonding and reactivity throughout the entire Periodic Table. Knowing actinide behavior allows us to exploit their properties for uses in energy production and environmental stewardship.
Researchers investigated the chemistry of protactinium to understand the impact of having the most energetic electrons in orbitals derived from mixing symmetries of the actinide 5f-electron orbitals and the transition metal 6d orbitals. They used exploratory chemical synthesis and quantum chemical calculations to understand the behavior of protactinium and its relationship to the other actinide elements and to its transition metal homologues. Exploiting the developed chemistry of polyoxometalate clusters, small transition-metal-oxide nanoparticles, the preparation and chemistry of a protactinium polyoxometalate was attempted. The result was a new structural motif that calculations demonstrate is unique to the actinide elements because of the availability of their 5f electrons. Quantum chemical calculations highlighted the changing participation of the 5f and 6d electrons across a series of actinide and transition metal polyoxometalates. These fundamental periodic changes in orbital character to the bonding in the early actinides have been previously predicted; although for the particular case of protactinium, we demonstrate a novel chemistry that cannot be predicted simply by summing relative portions of d and f character. The observed increases in 5f orbital participation demonstrate that protactinium occupies an important position in the Periodic Table for studying the changing influences of the 5f and 6d orbitals to the structure, bonding, and reactivity of actinide elements.
Argonne National Laboratory
This material is based upon work supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences, and was conducted at Argonne National Laboratory, operated for the Department of Energy. Computational resources have been provided by the Laboratory Computing Resource Center of the Argonne National Laboratory (Project MDCounterIons FY- 16) and the Laboratoire de Physique des Lasers, Atomes et Molécules cluster financed by the Agence Nationale de la Recherche through the Laboratory of Excellence Chemical and Physical Properties of the Atmosphere and the Ministry of Higher Education and Research, Hauts de France Council and European Regional Development Fund through the Contrat de Projets Etat-Region.
R.E. Wilson, S. De Sio, and V. Vallet, "Protactinium and the intersection of actinide and transition metal chemistry." Nature Communications 9, 622 (2018). [DOI: 10.1038/s41467-018-02972-z]
Argonne National Laboratory press release: The element of surprise