Electron Affinity of Actinide (IV) Carboxylate Complexes from MN12-L Density-Functional Calculations and Explainable Machine Learning.

We present a relativistic density-functional and explainable machine learning study of the vertical electron affinity (EA) of 14 homologous actinide(IV) carboxylate complexes [M(L)3]+ (M = Th-Lr; L = propionate, acrylate). All values were obtained at the MN12-L level via the def2-TZVPPD basis set for H, C, and O and a Stuttgart small-core relativistic effective-core potential, together with its associated valence basis, for the actinide. For every (metal, ligand) pair, all physically accessible An(IV) multiplicities were considered (52 calculations in total), and the ground state was assigned to the multiplicity with the lowest total electronic energy. The EA, evaluated as the negative of the lower α- and β-LUMO eigenvalues (the "true LUMO"), increases monotonically across the series from 5.53 eV at Th(5f0) to 10.80 eV at Lr(5f13) for the propionates (5.95 → 10.58 eV for the acrylates), with one outlier: at Th, the LUMO is forced into the 7 s/7p-ligand manifold because no 5f acceptor is available. Squared coefficient population analysis of the corrected ground-state wavefunctions confirms that for Pa-No, the LUMO is predominantly metal-5f (59%-82% on the metal), whereas at Lr, the highest-energy 5f spin orbital mixes strongly with the carboxylate π* manifold (5f ≈20%-28%, O ≈42%-50%). A nine-feature explainable machine-learning model trained on the corrected dataset reproduces the EA series under leave-one-metal-out cross-validation with R2 = +0.70 and MAE = 0.56 eV; permutation and SHapley additive exPlanations (SHAP) analyses identify the 5f-electron count and the atomic spin-orbit constant as the single dominant predictor cluster (Pearson r = +0.94 for the 5f count). The cross-ligand transferability is excellent (R2 = 0.85-0.88, MAE = 0.35-0.43 eV). The combined DFT-ML analysis provides a quantitative reference for An (IV) carboxylate EAs and isolates the 5f-shell occupation as the single physical lever governing their reduction propensity.