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

Four trivalent, <i>f</i>-element tetrazolate hydrate complexes [M(H₂O)₉](Hdtb)₃·<i>n</i>H₂O (<b>Nd1</b>, <i>n</i> = 7 and <b>Pu1</b>, <i>n</i> = 9; dtb^2- = 1,3-di(tetrazolate-5-yl)benzene) and [M(Hdtb)(H₂O)₈](dtb)·11H₂O (<b>Nd2</b> and <b>Pu2</b>) were prepared using metathesis reactions. These complexes contain hydrated M(III) cations, but in the latter complexes, <b>Nd2</b> and <b>Pu2</b>, 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 <b>Pu2</b>, 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 <b>Nd2</b>. 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 <b>Pu2</b> and the Nd(III)-N bond in <b>Nd2</b>. 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 <b>Pu1</b> and <b>Pu2</b> further support this observation, highlighting a surprising degree of similarity in their electronic structures.