New Infrared absorption data have been obtained using isolation in solid argon and neon on copper carbonyl molecules, CuCO and Cu(CO) 2 , two very reactive molecules. For CuCO, all three fundamentals are now observed and it presents an unusually large matrix effect, in line with former results indicating a weak metal carbon interaction and a large total dipole moment. With help of isotopic effects, new data indicates unambiguously that Cu(CO) 2 is linear and presents a notably stronger metal-carbon interaction than CuCO. Quantum chemical calculation have been carried out at the CCSD(T) level for determining energies and structural properties, as well as spectroscopic observables. The results enable an assessment of the high stability of the Cu(CO) 2 molecule (first bond dissociation energy of 66 kJ/mole) which suggest that observation at room temperature is possible and give a first evidence of Renner effect on a penta-atomic molecule.