The triphenylphosphane substituted derivatives Co-2(CO)(6)(CHCO2Et)(PPh3), [mu(2)-{ethoxycarbonyl(methylene)}-mu(2)-(carbonyl)-(tricarbonyl-cobalt) -(triphenylphosphane-dicarbonyl-cobalt) (Co-Co)] (3) and Co-2(CO)(5)(CHCO2Et)(PPh3)(2), [mu(2)-{ethoxycarbonyl-(methylene)}-mu(2)-(carbonyl)-bis(triphenylphosph ane-dicarbonyl-cobalt) (Co-Co)] (4) were prepared from the known Co-2(CO)(7)(CHCO2Et) (1) and Co-2(CO)(6)(CHCO2Et)(2) (2) complexes with triphenylphosphane. The new complexes 3 and 4 as well as Co-2(COMPPh3), Co-2(CO)(6)(PPh3)(2), and Co-2(CO)(6) (dppm) (dppm=Ph2PCH2PPh2) were found to be suitable catalyst precursors in the selective carbonylation of ethyl diazoacetate to diethyl malonate in the presence of ethanol. (c) 2005 Elsevier B.V. All rights reserved.