Department of Chemistry Seminar – Dr. David Clark

Speaker:

Host: Dr. Jim Boncella

Title: Applications of Ligand K-edge XAS to study the relative roles of 5f and 6d orbitals in actinide metal-ligand bonding

Abstract:

Despite decades of study, the degree to which f-block elements participate in covalent bonding is a topic of considerable discussion, and unraveling the relative roles of actinide and lanthanide valence f and d orbitals in chemical bonding has become an active area of experimental and computational study.  From MO theory, covalent bonding can be described via canonical mixing of M and L AOs resulting in an electron pair being shared in a bonding MO, and an energetic stabilization of the system.1,2  Covalency in the actinides is extremely challenging due to the radial contraction and AO energy decreases with Zeff across the series, and with variable metal oxidation states and ligand orbital energetics.  Of the experimental approaches to determine covalency, ligand K-edge X-ray Absorption Spectroscopy (XAS) has emerged as a powerful measure of the amount of ligand orbital mixing with metal-based orbitals.3  XAS studies have played a seminal role in reinvigorating the discussion of f-block covalency and attracting new experimental and theoretical techniques to aid our understanding over the last several decades.4,5

I will discuss applications of Cl and O K-edge XAS and relativistic electronic structure calculations for evaluating the relative roles of valence f and d orbitals in chemical bonding. We use the orbital mixing derived from XAS to assess covalency trends across a series of isostructural transition metal, lanthanide, and actinide molecules shown in Scheme 1.6-10  LA-UR-26-28502

1. Albright, Burdett, Whangbo, Orbital Interactions in Chemistry, 1985, Wiley, NY
2. Ballhausen, Introduction to Ligand Field Theory, 1962, McGraw-Hill, NY
3. Solomon et al., Coord. Chem. Rev. 2005, 249, 97
4. Denning et al.,  J. Chem. Phys. A, 2007, 111, 4125
5. Kaltsoyannis, Kerridge, Nature Rev. Chem 2024, 701
6. Kozimor et al, J Am Chem Soc 2009, 131, 12125.
7. Minasian et al, J Am Chem Soc 2012, 134, 5586.
8. Spencer et al, J Am Chem Soc 2013, 135, 2279.
9. Löble et al, J Am Chem Soc 2015, 137, 2506.
10. Su et al, J Am Chem Soc 2018, 140, 17977