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

The experimental CO stretching frequencies in the 1:1 adducts between (C 5 H 5− n R n ) 3 U and CO range from 1976 cm −1 in (C 5 H 4 SiMe 3 ) 3 U(CO) to 1900 cm −1 in (C 5 HMe 4 ) 3 U(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′ 3 U fragment composed of f σ and d σ 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′ 3 U 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.