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

A trivalent plutonium-pyrazinyl-tetrazolate complex Na[Pu(Hdtp)(dtp)(HO)]·9HO (Pu_dtp, Hdtp = 2,3-di-1-tetrazol-5-ylpyrazine) was synthesized through metathesis reaction of plutonium bromide and Na(dtp)·2HO in water. This structure is particularly notable among complexes formed by trivalent f-elements and the dtp ligand in aqueous media. In contrast to other trivalent f-elements, including all Ln (with the exception of Pm) and Cm, preferentially coordinated with eight water molecules rather than the nitrogen donors of the dtp ligand, Pu exhibits a distinct affinity for nitrogen coordination. This observation provides strong evidence that the 5f electrons in Pu 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).