Synthesis of Transition Metal Formyl Complexes

Transition metal acyls are often synthesized by acylation of a transition metal anion (24). Also, transition metal acyls (sometimes transient) can be 

All anionic transition metal formyl complexes described in the literature through the end of 1980 (21-47) are compiled in Table I. Since several of these have been prepared with more than one counterion, cations are not specified in the table. Geometric isomers are not assigned unless warranted by direct spectroscopic evidence. Also, the stability data in Table 1 should be regarded as qualitative, since decomposition rates have been shown to be dependent on both purity and counterion. When half-lives are specified, they are usually based upon measured rate constants. 

The first synthesis of a formyl complex was described in 1973 (27, 28). In a landmark paper, Collman and Winter reported that the reaction of Na2Fe(CO)4 with formic acetic anhydride [Eq. (4)] afforded the anionic formyl Na+(CO)4Fe(CHOU (subsequently isolated as its [(CeHs PjiN or PPN+ salt) in good yield. Formic acetic anhydride is an excellent formylat-ing agent. This seemingly unusual reagent was selected because many HCOX species (X = Cl, 02CH) are unstable at room temperature (4 ). 

Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis (29). One reason is that formylation of a transition metal monoanion would afford a neutral formyl complex. Insofar as comparisons are valid, neutral formyl complexes tend to be kinet-ically less stable than anionic formyl complexes. In cases where neutral formyl complexes are stable (vide infra), the corresponding transition metal monoanions are unknown. Whereas formic acetic anyhydride might be of greater use for the preparation of anionic formyl complexes from transition metal dianions, only a limited number of transition metal dianions [i.e., (CO)5Cr2 , (t7-C5H5)(CO)3V2 J are known (49). These appear to 

As mentioned above, appropriate hydride nucleophiles are capable of attacking coordinated CO [Eq. (3)]. This route to anionic formyl complexes was reported in 1976 by Casey, Gladysz, and Winter (29-34). All of the anionic formyl complexes in Table 1, including 22 [Eq. (4)], can be prepared by hydride attack on neutral metal carbonyl precursors. 

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