Theoretical Study on the Degree of CO(2) Activation in CO(2)-Coordinated Ni(0) Complexes

[Image: see text] The geometrical characteristic and the degree of CO(2) activation of the CO(2)-coordinated Ni(0) complexes were investigated computationally by quantum chemical means for bidentate and tridentate ligands of PP, PP(Me)P, and PNP, and sometimes with co-complexing Fe(II) to differently coordinate CO(2). 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 CO(2) 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 CO(2) activation than those of PP and PP(Me)P. In particular, Fe(II)-PNP in the CO(2)-bridged diatomic complex enhances CO(2) activation by another ∼20%, partly through the inductive effect of Fe(II), which pulls electron density from Ni-PNP across the CO(2)-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 CO(2)-bridged diatomic complex to develop more efficient homogeneous catalyst.