Covalency in Molecular Actinide Chemistry

  • Sophie Cooper

Student thesis: Phd

Abstract

After the first reported synthesis of californium borate in 2014, interest in the covalent behaviour of the later members of the actinide series was reignited. Unlike previous californium structures, which behave similar to their lanthanide counter- parts, this structure presented unexpected covalency; with the electron donation from ligand to metal involving both the 6d and 5f orbitals. Hence in this thesis, in an attempt to understand this behaviour and the general covalency trends of the later actinides, we developed a DFT method to assess the covalency across the series (from Th to No) using: Quantum Theory of Atoms (QTAIM); Natural Bonding Orbitals (NBO); Natural Resonance Theory (NRT); and subsequently, Natural Localised Molecular Orbitals (NLMOs) and Second Order Perturbation Theory Analysis. These methods were first tested on the AnCl3 (where An=Th- No), where we found that whilst the QTAIM bond critical point metrics indicates covalency decreases across the series, the bond order metrics indicates covalency plateaus/ moderately increases from Cm to Md. We believe that is due to the bond order metrics (unlike the bond critical point metrics) being able to measure both overlap driven and energy degeneracy driven covalency (a form of covalency that has been previous witnessed in the actinide series). The later members of the series have an increasingly stable 2+ oxidation state, therefore, with the use of AnCl2, we then went onto investigate the covalency trends of the divalent state. As the AnCl2 molecules behave less covalently than their AnCl3 counterparts at the end of the series, we suggest that the increased covalent behaviour seen with AnCl3 is not related to the presence of a stable divalent state but the greater energy match between ligand 3p and metal 5f orbitals. By also looking at various borate models we have discovered that despite the magnitude of covalency being related to the type of donor atom (with the AnCl3 molecules being consistently more covalent than their An(OB(OH)2)3 counterparts) the trends in covalency are more effected by the geometry of the ligand system.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorNikolas Kaltsoyannis (Supervisor)

Keywords

  • Covalency
  • Actinides
  • Computational Chemistry
  • Molecular DFT

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