The most invoked mechanism for ethenediolate formation from formyl intermediates is through a hydride insertion Mechanism A. This mechanism was proposed by Bercaw in their original work but with limited experimental or computational support for the steps following generation of the formyl intermediate. In 1991, Wolczanski and co-workers reported the reaction of CO with zirconium hydride complex [Zr(NHSi t Bu3)3H] (13) to form an ethenediolate species (Scheme 5). While in this case the formyl intermediate 14 could not be observed, its formation is underpinned by the discoveries with closely related Zr and Th metallocence systems described above. Nevertheless, data to support the formation of two downstream intermediates in the hydride insertion mechanism was collected. Specifically, the oxymethylene complex Andersen and co-workers reported the reaction of monomeric [Ce(Cp')2H] (18; Cp' = 1,2,4trimethylcyclopentadienyl) with CO yielded the corresponding cis-ethenediolate complex 21 (Scheme 6). Conducting the reaction in n-pentane solution led to precipitation of the bridging In 2015 Hill and co-workers and Jones, Stasch, Maron and co-workers simultaneously reported the reaction of a -diketiminate stabilised magnesium hydride reagent with CO energy barrier to give the bridged oxymethylene complex 33 (H ‡ = -15.8 kcal mol -1 ). A second CO insertion and subsequent 1,2-H shift yielded the identified ethenediolate complex through an overall exergonic reaction (H° = -82.9 kcal mol -1 from free 28 and CO).