Metal formyl complexes are often prepared by the reaction of metal carbonyls with hydride reagents:
:[Re(CO)]  +  H   →   (CO)ReCHO
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.
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. Decarbonylation leads to de-insertion of the carbonyl, yielding hydride complexes.