Covalency of M-N Bonds in Isomorphous Lanthanide and Actinide 5‑(2-Pyridyl)‑1H‑tetrazolate Complexes

Experimental and computational analyses of [M-(pdtz)₃(H₂O)₃]·3.5H₂O (M³⁺ = Pu³⁺-Cm³⁺, La³⁺-Nd³⁺, and Sm³⁺-Ho³⁺, pdtz^- = 5-(2-pyridyl)-1H-tetrazolate) were conducted to understand potential differences in bonding between lanthanide and actinide complexes with a N-donor ligand. Structural analyses show that the An-N bond distances in the Pu³⁺, Am³⁺, and Cm³⁺ complexes are within error of one another. Whereas in the lanthanide series, there is a nearly linear decrease in the Ln-N bond lengths from La³⁺ to Ho³⁺ (excluding Pm³⁺). The An-N bond lengths are ∼0.015 Å shorter than their similarly-sized lanthanide analogs, in agreement with computational results that suggest greater covalent character in these bonds versus those with lanthanides. QTAIM analysis indicates that the An-N orbital mixing remains essentially unchanged from Pu³⁺ to Cm³⁺, consistent with the nearly identical An-N bond lengths. However, upon deconvolution of the NLMOs into orbital compositions, the metal orbital contributions to An-N bonding decreases slightly overall wherein the 6d involvement remains constant, 7s involvement slightly increases, and 5f participation decreases. The molecular orbital energy diagram indicates that energy degeneracy between the 5f metal and 2p ligand orbitals increases from Pu³⁺ to Cm³⁺ and counteracts the contraction of the 5f orbtials. Together with prior reports of decreasing energy degeneracy between 5f and 3p orbitals from Np³⁺ to Cf³⁺, these observations provide guidance on understanding how chemical bonding evolves in the actinide series.