Transition metal formyl complex

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.^[2]

Structure of the formyl complex CpRe(PPh₃)(NO)CHO. Selected distances: d_HC-O = 122.1, d_HC-Re = 205.5, d_ON-Re = 177.7 pm.^[1]

Structure and bonding

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.

Synthesis and reactions

Metal formyl complexes are often prepared by the reaction of metal carbonyls with hydride reagents:^[3]

[Re(CO)₆]⁺ + H⁻ → (CO)₅ReCHO

The 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.^[4]

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.^[5] Decarbonylation leads to de-insertion of the carbonyl, yielding hydride complexes.^[2]