Cyclopentadienyldicarbonylmethyliron, [CpFe(CO)
2Me] (
1), undergoes migratory carbonyl insertion under the influence of isosteric phosphine ligands P(4-FC
6H
4)
3 and P(4-MeC
6H
4)
3. The products of the reaction, [CpFe(CO)(COMe)P(4-FC
6H
4)
3] (
2a) and [CpFe(CO)(COMe)P(4-MeC
6H
4)
3] (
2b), were characterised by X-ray crystallography. In both structures, the iron atom adopts a pseudo octahedral coordination geometry. Fe-P bond distances are the same at 2.1932(8) Å in
2a and
2b, respectively. Thus, contrary to what was expected, X-ray data could not be used to quantitatively differentiate between the two phosphine ligands in
2a and
2b. Therefore, additional spectroscopic techniques such as IR and NMR were employed. Similarly, the Fe-C bond lengths of the carbonyl (Fe-CO) and acetyl (Fe-COMe) are 1.748(3) and 1.955(3) in
2a, and 1.744(3) and 1.951(3) Å in
2b, respectively.The migratory carbonyl insertion was studied by NMR, IR, and UV-vis spectroscopies to determine the mechanism and the rate law. Results from NMR spectroscopy show that the formation of the product is accompanied by oxidation of the corresponding phosphine ligand. An increase in the reactivity of migratory carbonyl insertion for P(4-MeC
6H
4)
3 was observed when the solvent was changed from CH
2Cl
2 to MeCN. The kinetic data showed that P(4-MeC
6H
4)
3 reacts faster than P(4-FC
6H
4)
3.
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