Upcoming Events

Seminar: David Clark

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

October 21, 2026

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. 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. 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. 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. We suggest that while 6d-orbital mixing with ligand AOs is governed more by traditional overlap considerations, the 5f mixing is driven more by orbital energy neardegeneracy with ligand AOs across the actinide series. The latter does not lead to stronger bonds but fuels new discussion of chemical bonding and covalency in the f-block series.