Theoretical Study on the Degree of CO₂ Activation in CO₂-Coordinated Ni(0) Complexes

The geometrical characteristic and the degree of CO₂ activation of the CO₂-coordinated Ni(0) complexes were investigated computationally by quantum chemical means for bidentate and tridentate ligands of PP, PP^MeP, and PNP, and sometimes with co-complexing Fe(II) to differently coordinate CO₂. We show that the coordination geometry of the central metal is determined by the ligand geometry. The charge and the energy decomposition analyses show that the charge transfer energy through orbital mixing has a strong correlation with CO₂ net charge, while the binding energy cannot due to the lack of the coordination number and the deformation energy of the ligand. Among the examined ligands, PNP with negatively charged secondary amine makes Ni(0) an electron-rich atom, which results in an ∼20% higher CO₂ activation than those of PP and PP^MeP. In particular, Fe(II)-PNP in the CO₂-bridged diatomic complex enhances CO₂ activation by another ∼20%, partly through the inductive effect of Fe(II), which pulls electron density from Ni-PNP across the CO₂-bridge and partly by the backward donation from Fe(II)-PNP. Therefore, the present study encourages us to design a strongly electron-donating ligand and a CO₂-bridged diatomic complex to develop more efficient homogeneous catalyst.