Unveiling the Covalency of Versatile Pu(III)–N bonds in a Unique Plutonium(III) Complex

A trivalent plutonium-pyrizinyl-tetrazolate complex Na₂[Pu(Hdtp)(dtp)₂(H₂O)₄]·9H₂O (Pu_dtp, H₂dtp = 2,3-di-1H-tetrazol-5-ylpyrazine) was synthesized through metathesis reaction of plutonium bromide and Na₂(dtp)·2H₂O in water. This structure is particularly notable among complexes formed by trivalent f-elements and the dtp²⁻ ligand in aqueous media. In contrast to other trivalent f-elements, including all Ln³⁺ (with the exception of Pm³⁺) and Cm³⁺, preferentially coordinated with eight water molecules rather than the nitrogen donors of the dtp²⁻ ligand, Pu³⁺ exhibits a distinct affinity for nitrogen coordination. This observation provides strong evidence that the 5f electrons in Pu³⁺ are more delocalized than other studied trivalent f-elements, consistent with Hard-Soft Acid-Base theory. In Pu_dtp, three distinct Pu(III)–N bonds are present: one Pu(III)–N5 from pyrazinyl, one Pu(III)–N4 from the least electronegative nitrogen in the tetrazolate, and three Pu(III)–N1/N2/N3 from the most electronegative nitrogens in the tetrazolate. Experimental Pu(III)–N bond lengths, Wiberg bond indices (WBI), natural localized molecular orbitals (NLMO), Quantum Theory of Atoms in Molecules (QTAIM), and energy decomposition analysis (EDA), reveal a covalency trend: Pu(III)–N from the most electronegative nitrogen in tetrazolate > Pu(III)–N from the least electronegative nitrogen in tetrazolate > Pu(III)–N from pyrazinyl. This trend arises from the increased negative charge on the most electronegative nitrogen atoms in the tetrazolate ring, enhancing electrostatic Pu−N1/N2/N3 interactions. These stronger interactions promote better orbital overlap, leading to greater covalency compared to the less electronegative nitrogen in tetrazolate (Pu−N4) and the neutral pyrazinyl nitrogen (Pu−N5).