The Bond Between CO and Cp?3U in Cp?3U(CO) involves Backbondingfrom the Cp'3U Ligand-based Orbitals of ?pi-Symmetry, where Cp' Represents a Substituted Cyclopentadienyl Ligand.

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 play in 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 sigma-bond is formed between CO and an empty sigma-orbital on the Cp'3U fragment composed of f sigma and d sigma parentage orbitals. The backbonding interaction, which results in lowering the CO stretching frequency, does not originate from non-bonding metal-based orbitals but from the filled ligand-based orbitals of pi-symmetry that are used for bonding in the Cp'3U fragment. This model, which is different from the backbonding model used in the d-transition metal complexes, rationalizes the large substituent effect in the 5f-metal complexes