Reduction of the heterobimetallic .mu.-.eta.1(C):.eta.2(O,O')-CO2 complex Cp(CO)2Ru(CO2)Zr(Cl)Cp2 to its .mu.-.eta.1(C):.eta.1(O)-formaldehyde derivative Cp(CO)2Ru(CH2O)Zr(Cl)Cp2: hydride transfer occurs at ligated carbon monoxide