The experimental electron density (ED) of the triclinic phase of Co2(CO)6(mu-CO)(mu-C4O2H2), mu-carbonyl-mu-5-oxo-2,5-dihydrofuran-2-ylbis(tricarbonylcobalt), has been determined through X-ray diffraction at 120 K. The presence of a 'closed shell' Co-Co bond in the title compound, found in an experimental ED study of its orthorhombic form, is confirmed by the Quantum Theory of Atoms in Molecules. However, the two phases show a significant Co-Co bond length difference [triclinic: 2.4402 (2) A; orthorhombic: 2.4222 (3) A]. The flat distribution of the experimental ED along the Co-Co bond path and on the two Co2C rings allows for variations of the Co-Co bond length which may be easily induced by the different packing arrangements of the two forms.