Thermodynamic Effects on Migratory Aptitudes

In many cases, CO insertion reactions have been shown to be thermodynamically unfavorable. In these cases, de-insertion of CO occurs. For example, acetyhnetal complexes do not imdergo insertion to form stable a-dicarbonyl compounds. Instead, a-dicarbonyl compoimds de-insert CO to form metal-acyl complexes and free CO (Equation 9.27)P Likewise, the carbonylation of perfluoroalkyl transition metal complexes is usually uphill thermodynamically. Decarbonylation of perfluoroacetyl complexes is one method to prepare perfluoroalkyl complexes (Equation 9.28).  

For similar reasons, CO rarely inserts into metal hydrides. The M-H bond energy is usually high enough that insertion to form a formyl derivative is tirermodynamically uphill. Therefore, decarbonylation of formyl ligands is more typical (Equation 9.29), and formyl complexes of the transition metals are usually prepared by indirect means.  

A few exceptions to the imfavorable tirermodynamics for insertion of CO into transition metal hydrides have been observed. One exception is shown in Equation 9.30. The favorable thermodynamics must result from an unusually strong bond between the formyl carbon and rhodium. A second exception is shown in Equation 9.31, wherein an 

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