Physical Origin and Periodicity of the Highest Oxidation States in Heavy-Element Chemistry

ConspectusThe exploitation of combustion of materials stands as a cornerstone of early human development. A theoretical framework for this field began to take shape only around 1700 and finally achieved a sound foundation through the electronic quantum-chemical oxidation concepts of the 20th century. Eventually, a decade ago the IUPAC defined the essential rules for a unique definition of atomic oxidation states (OS) or OS numbers in molecules and extended structures. In specific cases, however, these rules need be tailored to specific chemical observations, including amazing experiences with the heaviest elements. We review our findings and updated interpretations, particularly with regard to previously unexpected trends in the highest oxidation states (HOS) of heavy atomic compounds.The HOS is a qualitative integer parameter for characterizing the chemistry of an element. It is determined by three basic factors: (i) by the number <i>g</i> of loosely bound electrons in the atom's valence shell, usually related to its group number <i>G</i> in the periodic table (<i>g = G</i> mod 10, with mod meaning <i>g</i> = 2 to 8 for <i>G</i> = 12-18); (ii) by the fraction of these <i>g</i> electrons that can be chemically activated and may fall below <i>g</i> toward the end of a series; and (iii) by the local and long-range "environmental" conditions (mainly the interaction strengths of the ligands), which range from those in normal life, laboratories and industry, to more extreme conditions such as deep inside the Earth, outside in cosmic space, or in special laboratories.In recent years, our group has systematically investigated the regularity of the HOS over the periodic table, in particular the electronic structures of compounds of various d- (transition metals) and f-block elements (lanthanides and actinides), applying both density-functional approximations and more advanced quantum-chemical methods. Details of the HOS values in the border ranges of feasibility, depending on thermodynamic environment and ligand-interaction capabilities, are conceptually analyzed, incorporating also discoveries of other researchers. The patterns of the HOS from the light s- and sp- to the heavy d- and f-block elements are elucidated, showing systematic deviations from simple textbook rules. The particularly large and small Core-Valence (CV) orbital-energy gaps at, respectively, the upper-right and lower-left corners of the periodic table interrupt the regular trends of the HOS of the sp-block elements, causing HOS < <i>g</i> (as for O or Pt) or HOS > <i>g</i> (as for Cs or Ra). On the other hand, the d-transition metals (TM) and also the actinide 5f-series (An) in the middle of the periodic table show a more uniform variation of the HOS with one common maximum at 8(±1), while the lanthanide 4f-series (Ln) shows two lower HOS maxima, due to their peculiarly small 4f-orbital radii and varying energies.Our report is intended both as a summary of knowledge, as a distillation of new insights, and as a roadmap for further research into the oxidation phenomena.