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.
The effect of cyclic and aromatic substituents on the
complexation
behavior of phosphine oxide ligands with Am(III) and Eu(III) was investigated
at density functional theory (DFT) and domain-based local pair natural
orbital coupled-cluster (DLPNO–CC) levels. Combining DFT with
accurate coupled cluster methods, we have evaluated the dispersion
energy contributions to the complexation energies for trivalent Am
and Eu complexes for the first time. Irrespective of the nature of
substituents on the P atom, the electronic structure of the PO
group remains identical in all of the ligands. The study reveals the
importance of dispersion interactions during complexation and is estimated
to be more significant for Am(III) than for Eu(III) complexes.
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