The active site of the [NiFe]-hydrogenase contains a pentacoordinated iron atom; therefore, a vacant coordination site is available for substrate binding. Nonetheless, most organometallic models of the [NiFe]hydrogenase failed to reproduce this key feature of the active site. In order to rationalize such behavior, quantum chemical calculations were carried out on a series of [Fe(CN)(2)(CO)(n)(dithiolate)(2-) n = 1,2 complexes, where dithiolate denotes the ligands (CF3)(2)C2S22-, (CO2Me)(2)C2S22-, Ph2C2S22-, C6Cl2H2S22-, C6H4S22-, C2H4S22-, and C3H6S22-. Structural and energetic features are discussed, and a topological analysis based on two scalar fields, the one-electron density and the electron localization function (ELF), has been attempted to describe the nature of the metal-ligand bonds. The present approach contributes to better understand the ability of noninnocent dithiolene to strongly labilize one CO whereas innocent dithiolate cannot. The methodology developed throughout the paper could be useful in the field of the CO-releasing molecules.