Abstract The reactivity of low-coordinated anions of MgO towards CO has been studied by means of ab initio cluster model DFT calculations. Differently from MgO terraces, where the O 2− anions are totally unreactive, oxide anions at step, edge and corner sites exhibit a very high basicity and reactivity towards CO. The reaction: [O LC(CO) n−1 ] 2− (s) + CO (g) → [O LC(CO) n ] 2− (s) leads to the formation of polymeric (C n O n+1 ) 2− species, where n goes from 1 to 5. The reaction is non-activated and exothermic; only the formation of the tetramer appears to be nearly thermoneutral. The formation of the hexamer ( n = 6) is thermodynamically unfavorable. For each value of n, several isomers have been considered. The vibrational properties of the [O LC(CO) n ] 2− surface complexes have been computed and used to interpret the low-temperature infrared spectra obtained in the group of Zecchina [G. Spoto, E.N. Gribov, G. Ricchiardi, A. Damin, D. Scarano, S. Bordiga, C. Lamberti, A. Zecchina, Prog. Surf. Sci. 76 (2004) 71].