Salts of the anion [Co(COOCH3)2(CO)3]- with Cs+ and K+ (with the latter cation complexed by dibenzo-18-crown-6 ether) have been isolated and structurally characterized. In the trigonal bipyramidal cobaltate two C-coordinated COOCH3 groups occupy trans-axial positions. There are ionic interactions between the cation and the terminal oxygen atoms of methoxycarbonyl involving both of the coordinated COOCH3 groups in the case of the Cs salt but only one of them in the case of the K salt. One methoxy group of the anion [Co(COOCH3)2(CO)3]- is strongly nucleophilic, as shown by the reactions with Co2(CO)8 or CO2. Under an inert atmosphere [Co(COOCH3)2(CO)3]- undergoes elimination of dimethylcarbonate.