In organometallic chemistry, a transition metal formyl complex is a metal complex containing one (usually) or more formyl (CHO) ligand. A subset of transition metal acyl complexes, formyl complexes can be viewed as metalla-aldehydes. A representative example is (CO)5ReCHO. The formyl is viewed as an X (pseudohalide) ligand. Metal formyls are proposed as intermediates in the hydrogenation of carbon monoxide, as occurs in the Fischer-Tropsch process.[1]
The MCHO group is planar. A C=O double bond is indicated by X-ray crystallography. A second resonance structure has a M=C double bond, with negative charge on oxygen.
Metal formyl complexes are often prepared by the reaction of metal carbonyls with hydride reagents:[2]
[Re(CO)<sub>6</sub>]+ + H− → (CO)5ReCHOThe CO ligand is the electrophile and the hydride (provided typically from a borohydride) is the nucleophile.
Some metal formyls are produced by reaction of metal carbonyl anions with reagents that donate the equivalent of a formyl cation, such a mixed formate anhydrides.[3]
Metal formyls participate in many reactions, many of which are motivated by interest in Fischer-Tropsch chemistry. O-alkylation gives carbenoid complexes. The formyl ligand also functions as a base, allowing the formation of M-CH=O-M' linkages.[4] Decarbonylation leads to de-insertion of the carbonyl, yielding hydride complexes.[1]