Reverse reactions, for CO migration and alkyl

FIGURE 14-9 Mechanisms of Reverse Reactions for CO Migration and Alkyl Insertion (1). C indicates the location of... 

The benzoyl derivative, PhCOMn(CO)5 and related acyl derivatives have intrinsic interest because they can be prepared by alkyl group migration to coordinated CO, a formal internal nucleophilic attack. This reaction, which is often referred to as carbonyl insertion86), is reversible in certain instances to give the metal alkyl. The enthalpy of disruption for PhCOMn(CO)s (Table 13) can be divided up to give the b.e.c of the Mn-COPh bond if the values of T (Mn-CO) shown in Table 13 are used, then the Mn-COPh enthalpy lies in the range 105 10 kJ mol-1, the lower value being preferred as outlined earlier. 

The validity of this mechanism has been demonstrated for a number of CO insertion reactions. Thus when chemists use the term CO insertion, they usually mean alkyl migration. Several things to keep in mind when considering a reaction of this type are (I) it involves ligands which are cis to one another. (2) in the course of the reaction a vacant coordination becomes available, and (3) the reverse reaction cannot occur unless a ligand is first eliminated. 

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