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The Inverse Trans Influence in U(IV/V) complexesKöhler, L.; Patzschke, M.ORC; März, J.ORC; Stumpf, T.
The inverse trans influence (ITI) is an effect well-known to occur in high valent U(V/VI) complexes. It appears as a shortening of the M–L bond in trans position to a strongly donating ligand. The effect can be explained by electron density donation from the strong ligand to the metal center, which fills up the electron hole formed through electron density transfer from semi-core 6p to vacant 5f orbitals.[1,2] This results in the observed contraction and strengthening of the bond trans to the donating ligand.
To compare the ITI in UIV and UV complexes, the U compounds [UIV(L¹)₂(TMSA)Cl₃] 1 (L¹ = iPr₂Im, TMSA = (N(SiMe₃)₂)−) and [UV(TMSI)Cl₅](L¹H)₂ 2 (TMSI = NSiMe₃−) were synthesized from UIVstarting material in the presence of iPrIm. In the case of 1 the metal center is surrounded by three chloro-, two carbene and one TMSA ligand, whereas 2 exhibits five chloro- and one TMSI ligand, thus generating an dianion, whose charge is compensated by the protonated carbene. In 1, the U–Cl bond, located trans to the TMSA ligand, is remarkably shorter (0.02 Å) than the other U–Cl bonds. This indicates the existence of an ITI, induced by TMSA. Based on the higher valent UV cation and stronger donating TMSI ligand, a similar or stronger effect should also be observed in 2. Surprisingly, this could not be confirmed by the experimental data. The U–Cl bond lying trans to the TMSI ligand is not the shortest U–Cl bond (2.68 Å compared to 2.66 Å for U–Cl1/2), but is in the same range as the other chloro ligands. The absence of a notable ITI can be attributed to intermolecular interactions in the crystal structure of 2. Structure optimization of the molecular UV complex dianion by DFT yield a U–Cl5 bond length of 2.55 Å, shorter than any other U–Cl bond by 0.02 Å. The difference between experiment and theory results from a great number of electrostatic interactions and hydrogen bonding between the complex dianion and the carbene counterions in 2. Similar intermolecular interactions are not present in the crystal structure of 1, which is why the ITI could be observed for this compound.
The results demonstrate that the ITI affects complex structures for both, UIV and UV compounds, but additional effects, such as the intermolecular network observed in the structure of 2 can surpass its relatively small structural contribution.
 M. Gregson, E. Lu, D. P. Mills, F. Tuna, E. J. L. McInnes, C. Hennig, A. C. Scheinost, J. McMaster, W. Lewis, A. J. Blake, et al., Nat. Commun. 2017, 8, 14137.
 B. Kosog, H. S. La Pierre, F. W. Heinemann, S. T. Liddle, K. Meyer, J. Am. Chem. Soc. 2012, 134, 5284–5289.
Keywords: inverse trans influence, ITI, uranium(V), carbenes,
Journées des Actinides, 14.-18.04.2019, Erice, Italien