DFT calculations were carried out for the Ni(0) complex Ni(PH3)(2)(eta(2)-CO2), which is a model compound for the well-known Ni(0) carbon dioxide complexes containing various tertiary phosphane ligands. The electronic structure of the complex was elucidated using domain-averaged Fermi hole (DAFH), quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), charge decomposition analysis (CDA), and natural bond orbital (NBO) methods. The carbon dioxide ligand in the complex reveals an unexpected coordination behavior. Apart from the expected pi-donation interaction, the C-O sigma bond takes also part in the electron donation. Moreover, the back-donation is slightly influenced by the phosphorus atom adjacent to the noncoordinated O of carbon dioxide as it transfers electron density directly to carbon. This unconventional way of back-donation may also explain the bent character of the Ni-C bond path. Due to excess kinetic energy density, no bond critical point was found between the coordinating oxygen and the nickel center. A strong relationship has been found between the DAFH and the NBO methods, which can provide additional information for the interpretation of DAFH eigenvectors.