Unveiling the covalency of versatile Pu(iii)-N bonds in a unique plutonium(iii) complex

A trivalent plutonium-pyrazinyl-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 dtp^2- 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 dtp^2- 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. 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 electrostatic interactions lead to shorter bond lengths, thereby enhancing orbital overlap and greater covalency, compared to the less electronegative nitrogen in tetrazolate (Pu-N4) and the neutral pyrazinyl nitrogen (Pu-N5).