The stability of carbonyl complexes of the group 11 chlorides CIMCO (M = Cu, Ag, Au) is investigated at the ab initio level using relativistic and nonrelativistic energy adjusted pseudopotentials for the metal atoms. The gold compound ClAuCO is relativistically stabilized resulting in an increase of the ClAu-CO dissociation energy of ca. 120 kJ/mol at the coupled cluster level (CCSD(T)). This explains the unusual stability of this compound compared to its lighter congeners observed by experimental methods. Structural data and vibrational frequencies are predicted. Our calculated results agree well with the few measured molecular properties available. For example, the calculated ClAu-CO dissociation energy of 182 kJ/mol is close to Calderazzo’s estimate of 200 kJ/mol. The unusual blue shift of the CO stretching frequency measured in ClAuCO compared to free CO is, however, not due to the effects of relativity as might he expected. A MO analysis shows that both metal-d and metal-p contributions are important in metal-ligand bonding in contrast to the interpretation given from Mossbauer data. Electric field gradients are discussed which are very sensitive to relativistic effects and to CO coordination on the metal center.