If alkoxydienyl complexes of the type [Pt(dieneOR)Cl] 2 are allowed to react with carbon monoxide under mild conditions, unstable carbonyl complexes are formed. The carbonyl complexes may react with an alcohol to give alkoxycarbonyl complexes of the type (diene)PtClCOOR. This reaction involves nucleophilic attack of alcohol on the coordinated carbonyl group, and expulsion of the alkoxyl group from the dienyl moiety with formation of a coordinated carboncarbon double bond. This reaction, however, is not of general occurrence. A comparison is made between the electrophilicities of coordinated carbon monoxide and coordinated carboncarbon double bond.