Experimental electron density in the triclinic phase of Co-2(CO)(6)(mu-CO)(mu-C4O2H2) at 120 K

The experimental electron density (ED) of the triclinic phase of Co-2(CO)(6 )(mu -CO)(mu -C4O2H2), mu -carbonyl-mu -5-oxo-2,5-dihydrofuran-2-ylbis(tric arbonylcobalt), 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 Quantu m Theory of Atoms in Molecules. However, the two phases show a significant Co-Co bond length difference [triclinic: 2.4402 (2) Angstrom; orthorhombic: 2.4222 (3) Angstrom]. The flat distribution of the experimental ED along t he Co-Co bond path and on the two Co2C rings allows for variations of the C o-Co bond length which may be easily induced by the different packing arran gements of the two forms.