Resolving the inner versus outer component of the 5 f radial wavefunction in the early actinides

Abstract

Actinide chemistry is largely defined by 5f orbital behaviour, but experimental measurements of their role in bonding are scarce, despite their importance for understanding chemical reactivity. Here, we demonstrate that M₄ -edge 3d4f resonant inelastic X-ray scattering can discern the averaged 5f radial wavefunction, revealing how 5f electron density varies with distance from the nucleus. This approach enables differential quantification of how the inner and outer components of the 5f radial wavefunction respond to ligand bonding. Applied to a series of actinide hexachloride complexes, [AnCl₆]^2- (An = U, Np, Pu), it is shown that bonding-induced expansion of the 5 f radial wavefunction increases along the series. However, this expansion is non-uniform; the outer region expands more when going from U to Pu than the inner region, which remains constrained by increasing nuclear charge. This identified sensitivity is used to rationalise 5f covalency trends across the early actinides. The results provide needed experimental insight into the mechanisms by which early actinide 5f orbitals engage in covalent bonding.