In 1984, Bercaw and co-workers reported reaction of 6, 2 , and H2 to give the cisethenediolate product 8 (Scheme 3). In efforts to stabilise a proposed formyl intermediate, ). This seminal work is an exceedingly rare example in which formyl and ketene intermediates are identifiable and characterised during carbon chain formation from CO and hydride sites and the fundamental steps are related to Mechanism C for ethenediolate formation. Scheme 13. Reaction of zirconium hydride complex 12 with zirconocene carbonyl complexes 63-64. Formyl (65-67) and ketene (68-69) intermediates were identified during ethenocycle (70) formation.