Rationalizing the DCD Model in Transition Metal Carbonyls: A Conceptual Density Functional Theory Analysis

The bonding in transition metal carbonyls is discussed through the Dewar-Chatt-Duncanson (DCD) model of σ-donation from the ligand and π-back donation from the metal. However, there are no reports of direct quantification of the donation and back donation. Whenever it comes to the aspect of electron transfer, the fundamental concepts that are important are ionization energy (I), electron affinity (A), electronegativity (χ), hardness (η), and electrophilicity (ω). The global reactivity indices are calculated using conceptual density functional theory (CDFT). It was found that the back bonding and hence the experimental CO stretching frequency provide excellent correlation with I, A, and χ with r² values of 0.963, 0.903, and 0.965, respectively. While in correlation to η, two categories are developed in correlation to ν_CO. However, the best correlation is achieved from the local electrophilicity description of the multiphilic descriptor (Δω_M). Finally, the directional approach of the back donation is tackled by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) method, considering CO as one fragment and the rest as the other. A very good correlation to ν_CO is found with r² = 0.964. The back-bonding aspect is also explained from the second-order perturbation energy term as obtained from the natural bond orbital analysis. These correlations remain valid upon changing the functional and basis sets. In addition, considering Sc(CO) as the starting complex, hydrogen molecules are added to obtain Sc(CO)(H₂)_n (n = 1-5) complexes. In these complexes, the Kubas-type interactions are studied employing ETS-NOCV and quantum theory of atoms in molecules (QTAIM) analyses.