The new and unexpected organometallic mercury compound 3 has been identified in the palladium-catalyzed reaction between bis(ferrocenyl)mercury (1) and dimethylacetylenedicarboxylate (2) along with the expected dimethyl-(Z)-2,3-di(ferrocenyl)-2-butenedioate (4) and biferrocene (5). Organometallic mercury complex 3 resulted from syn-addition of 1 to 2 equiv of 2. Complex 3 has been characterized using NMR spectroscopy, while its structure was determined by X-ray crystallography. Complex 3 represents a rare example of an organomercury compound in which a mercury ion is coordinated to two vinylic fragments. Accurate, low-temperature experimental data for the crystal structure of 5 is also reported.