Strong donor ligands orient opposite each other in lanthanide and actinide complexes

In square planar or octahedral inorganic complexes, strong electron donor ligands typically prefer to bind in a cis rather than trans orientation relative to each other. But chemists have observed the opposite in a few actinide complexes in which strong donor ligands prefer a trans orientation and seem to reinforce each other. A new study suggests that situation may be more common than realized (Nat. Commun. 2017, DOI: 10.1038/ncomms14137). Previously, researchers had observed trans orientations only in linear, high-valent actinyl complexes with hard ligands, such as UO₂²⁺. In the new work, a team led by Stephen T. Liddle of the University of Manchester and Andrew Kerridge of Lancaster University synthesized cerium(IV), uranium(IV), and thorium(IV) bis(carbene) complexes with linear C=M=C cores and surprisingly short C=M bonds. The effect likely occurs because lanthanide 5p and actinide 6p orbitals can transfer electrons to the 4f and 5f orbitals, respectively, creating electron holes that may be filled via electron donation from trans ligands more readily than from cis ligands. The results suggest that the phenomenon may play a broader role than realized in the structure and reactivity of f-block complexes.