Subsequent work detailed the reduction of zirconium-coordinated CO ligands with 2 (Scheme 3). Reaction of [Zr(Cp*)2(CO)2] (6) with 2 under an atmosphere of H2 yielded the cis-isomer of ethenediolate complex 8. It is noteworthy that the analogous reaction in the absence of H2 gave a mixture of products. The reaction was predicted to proceed via a chelating formyl ligand bridging the two zirconium centres (7). A series of metal carbonyls (M = W, Cr, Mo, Nb) were shown to react analogously with 2, providing isolatable bridging formyl complexes but no C-C coupled products. Scheme 3. Reaction of zirconocene carbonyl (6) and hydride (2) complexes to provide cisethenediolate complex 8.