In the presence of 1 mol% octacarbonyl dicobalt ethyl diazoacetate can be transformed at room temperature and carbon monoxide pressure selectively into diethyl 2-diazo-3-oxo-pentanedi carboxyl ate or in the presence of an alcohol (methanol, ethanol, tert-butanol), phenol or diethylamine into the corresponding malonic acid derivatives in high yields. Ethoxycarbonyl-carbene-bridged dicobalt carbonyl complexes [mu(2)-{ethoxycarbonyl(methylene)-mu(2)-(carbonyl)-bis(tricarbonyl-cobal t) (Co-Co)] and [di-mu(2)(ethoxycarbonyl(methylene)}-bis(tricarbony-cobalt)(Co-Co)] proved to be intermediates in the catalytic reactions. (C) 2003 Elsevier Science B.V. All rights reserved.