Homoleptic binuclear chromium carbonyls: why haven't they been synthesized as stable molecules?

Density functional theory has been used to explore possible homoleptic binuclear Cr carbonyls Cr2(CO)n (n = 11, 10, 9, and 8) using the pure DFT method BP86 and the hybrid Hartree-Fock DFT method B3LYP. The binuclear Cr2(CO)11 is computed to be thermodynamically unstable with respect to dissociation into mononuclear fragments. Optimized structures with metal-metal multiple bonding are found for Cr2(CO)10 (triplet), Cr2(CO)9, and Cr2(CO)8.