The accurate experimental electron density of crystalline bis(5-cyclooctadiene)nickel, Ni(COD)(2), has been determined by X-ray diffraction (T = 125 K, 17 051 reflections measured for 2 theta less than or equal to 96 degrees) and it has been interpreted in terms of quantum theory of atoms in molecules (QTAM). The data were measured with a CCD area detector, whose performances were tested. This experimental electron density study of a pi-ligand eta(2)-coordinated to a metal atom is intended to test the Dewar-Chatt-Duncanson (DCD) bonding formalism providing further information about the pi-complex vs metallacycle dichotomy. QTAM analysis, within the NiC2 triangle, shows a ring structure with a bond critical point between the two C(sp(2)), two bond critical points between Ni and each carbon, and one ring critical point in its center. Topology speaks for a pi-complex with a concave ring structure and the overall bonding picture is in agreement with the DCD model: sigma-donation and pi-back-bonding are recognized in the Ni-C bond paths, which are inwardly curved (sigma-donation) but well separated (pi-back-donation).