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The electronic structure and complexation behavior of methylsubstituted phosphinic acids with U(VI) and Pu(IV) were explored by applying quantum chemical methods. In contrast to Ingold's classification, our results indicate that the methyl group is electron-withdrawing, reducing the phosphoryl group electron density in substituted phosphinic acids. The magnitude of the computed complexation energy values increases along with the series, PA → MPA → DMPA, and MP → MMP → MDMP, implying an increasing complexation tendency upon methyl group substitution for both U(VI) and Pu(IV) complexes. One of the nitrate groups in UO 2 (NO 3 ) 2 •2L complexes (L = PA, MPA, and DMPA) is in monodentate coordination mode due to the additional stability gained from O 2 N−O•••H hydrogen bonding interactions with the acidic H atoms of respective ligands. The calculation indicates marginally stronger metal−ligand interactions in Pu(IV) complexes compared to that in U(VI), which is supported by the computed complexation energies, M−O P bond lengths, ν(P�O), the extent of metal−ligand charge transfer, and properties of M−O P bond critical points. The energy landscape of substituted phosphinic acid ligands is further analyzed within the framework of the activation strain model to explain the energetic preference of certain conformers.

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