The simplest way to combine CO and H -on a metal surface results in the formation of a formyl ligand (CHO -). Two related mechanisms can be imagined for formyl generation: insertion of CO into a polarised metal-hydride bond, or nucleophilic attack of a hydride at a coordinated carbonyl ligand. Metal-formyl complexes have been proposed as intermediates in FT processes for both homo-and heterogeneous systems. Despite their importance, the detailed study of transition metal formyl complexes has been hampered by their low stability. Early studies established that these species undergo facile -elimination to form the corresponding metal-hydrido carbonyl complex. This reaction is potentially reversible, though in most cases the hydrido-carbonyl is thermodynamically favoured. The first structurally characterised formyl complex, [ReCp(CHO)(NO)(PPh 3 )], was reported by Gladysz and coworkers in 1979. We and others have shown that the formyl ligand can be stabilised through resonance with the corresponding oxycarbene structure (Figure ). Figure . Canonical forms of a metal-formyl complex and reported structural motifs formed via metal-formyl intermediates.