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

TL;DR

This study compares the bonding in lanthanide and actinide complexes with a nitrogen-donor ligand, finding that actinide bonds show more covalent character.

Contribution

The paper reveals how bonding differences arise in actinide complexes due to changes in orbital interactions and energy degeneracy.

Findings

Actinide–nitrogen bond lengths are shorter than lanthanide analogs, indicating greater covalency.

Orbital contributions to An–N bonding show decreasing 5f participation and increasing 7s involvement from Pu³⁺ to Cm³⁺.

Energy degeneracy between 5f and 2p orbitals increases, counteracting 5f orbital contraction in the actinide series.

Abstract

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.

Figures