Elucidation of an Unusually Long Pu-N Bond in a Plutonium(III)-Tetrazolate Complex

Four trivalent, f-element tetrazolate hydrate complexes [M(H₂O)₉](Hdtb)₃·nH₂O (Nd1, n = 7 and Pu1, n = 9; dtb^2- = 1,3-di(tetrazolate-5-yl)benzene) and [M(Hdtb)(H₂O)₈](dtb)·11H₂O (Nd2 and Pu2) were prepared using metathesis reactions. These complexes contain hydrated M(III) cations, but in the latter complexes, Nd2 and Pu2, one of the water molecules has been displaced by a long interaction between the M(III) cation and a Hdtb^- anion. Notably, the Pu(III)-N bond in Pu2, representing the longest ^IXPu(III)-N (^IX = nine coordinate) bond reported has a length of 2.8338(15) Å and is slightly shorter than the Nd(III)-N bond length of 2.8425(13) Å in Nd2. Analysis of bond lengths, Wiberg bond indices (WBI), natural localized molecular orbitals (NLMOs), and quantum theory of atoms in molecules (QTAIM) reveals that the metal contribution to the Pu(III)-N bond is marginally greater than that of the Pu(III)-OH₂ bonds in Pu2 and the Nd(III)-N bond in Nd2. Thus, this rather long M-N interaction provides an example where the expectation that An(III) compounds exhibit greater covalency with soft donor ligands compared to harder ligands fails. The absorption spectra of Pu1 and Pu2 further support this observation, highlighting a surprising degree of similarity in their electronic structures.