Electrophilic addition of a proton to 1,2-bis(carbomethoxy)acetylene inserted into the iron-iron bond of [(.mu.-SCH3)Fe(CO)(P(CH3)3)2]2. X-ray structure at -162.degree.C of the reaction product, a chelated 1,2-bis(carbomethoxy)ethylene complex, [((.mu.-SCH3)Fe(CO)(P(CH3)3)2)2(CH3COOCCHCOOCH3)][B(C6H5)4]