Despite their limited stability, metal formyl complexes have been invoked in the formation of several C2 and C3 products (Figure ) through further reactions with CO and H2 (or hydride sites). The most prevalent products are cis-ethenediolate complexes. These contain a {C2H2O2} 2-ligand whose coordination mode is dependent upon the nature of the metal centre and supporting ligand.  1 ,  2 , and η 4 motifs are known (Figure ). The related transethenediolate complex is markedly less common. Higher C3+ homologues are rare, with a single example of a C3 propanetriolate {C3H3O3} The first suggestion of metal-formyl complexes as intermediates in CO homologation relevant to the FT process was reported almost simultaneously by Casey and Bercaw in 1976. Casey and co-workers reported a metal-formyl complex formed by the reaction of a polarised metal-hydrogen bond with a metal carbonyl complex. Reaction of a series of Fe, Cr and W carbonyl complexes with trialkoxyborohydrides [HB(OR)3] -furnished the corresponding metal formyl complexes, identified by characteristic low-field resonances in the 1 H and 13 C NMR spectra. Though no direct C-C bond formation was observed, the authors speculated on the role of formyl complexes as potential intermediates in the first step of catalytic CO homologation with H2. At the same time, Bercaw and co-workers were investigating (Scheme 2). The putative intermediate 4 was not observed spectroscopically or characterised.