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The Bond between CO and Cp′3U in Cp′3U(CO) Involves Back-bonding from the Cp′3U Ligand-Based Orbitals of π-Symmetry, where Cp′ Represents a Substituted Cyclopentadienyl Ligand

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journal contribution
posted on 13.07.2009 by Laurent Maron, Odile Eisenstein, Richard A. Andersen
The experimental CO stretching frequencies in the 1:1 adducts between (C5H5−nRn)3U and CO range from 1976 cm−1 in (C5H4SiMe3)3U(CO) to 1900 cm−1 in (C5HMe4)3U(CO). The origin of the large difference between the stretching frequencies in free (2143 cm−1) and coordinated CO and the large effect the substituents on the cyclopentadienyl ligands have on the difference is explored by DFT calculations with a small core effective core potential in which 32 electrons on uranium are explicitly treated. The results of these calculations, along with a NBO analysis, show that a σ-bond is formed between CO and an empty σ orbital on the Cp′3U fragment composed of fσ and dσ parentage orbitals. The back-bonding interaction, which results in lowering the CO stretching frequency, does not originate from nonbonding metal-based orbitals but from the filled ligand-based orbitals of π-symmetry that are used for bonding in the Cp′3U fragment. This model, which is different from the back-bonding model used in the d-transition metal complexes, rationalizes the large substituent effect in the 5f-metal complexes.