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Actinide chemistry is largely defined by 5 f orbital behaviour, but experimental measurements of their role in bonding are scarce, despite their importance for understanding chemical reactivity.Here, we demonstrate that M 4 -edge 3d4 f resonant inelastic X-ray scattering can discern the averaged 5 f radial wavefunction, revealing how 5 f electron density varies with distance from the nucleus.This approach enables differential quantification of how the inner and outer components of the 5 f radial wavefunction respond to ligand bonding.Applied to a series of actinide hexachloride complexes, [AnCl 6 ] 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.The results provide needed experimental insight into the mechanisms by which early actinide 5 f orbitals engage in covalent bonding. MainThe physical and chemical properties of the actinides are influenced by the extent of 5 f orbital involvement in chemical bonding. [1][2]2][3] Actinide coordination chemistry research is rapidly expanding, revealing rich and varied chemistry including an increasing diversity of transuranium studies, 4-12 multiple bonds, [13][14][15] wide-ranging redox behaviour, [16][17][18] and catalysis. 19,20 n much of this work, the degree of covalency present in actinide ligand bonds underpins the interpretation of physicochemical properties.The proposed presence of 5 f metal-ligand covalency dates back to work by G. T. Seaborg and co-workers who rationalized ion-exchange behavior in early actinides as evidence of covalent mixing between actinide 5 f and Cl 3p orbitals. 21 notion was supported further by Streitwieser and colleagues following the seminal discovery of the actinocenes. 22In more recent years, actinide computational chemistry studies have exposed deeper questions concerning 5 f covalency, since different bonding analyses provide contrasting perspectives on both the degree of covalency present and the mechanism by which its