Kinetic Effects on Migratory Aptitudes

When the thermod3marrucs for insertion are favorable, the relative rates for insertion and de-insertion of CO into different alkyl, aryl, and benzyl groups is controlled by kinetic factors. Several trends have been observed. 

CpMo(CO)3CH2CeH4X + PPhg — CpMo(CO)2(PPh3)[C(0)CH2C6H4X) 

Re(Ph)[C(0)Me]/Re(Me)[C(0)Ph] is 50. However, migration of the methyl group occurs about 30 times faster than migration of the phenyl group. 

the rate of carbonyl insertion as a function of the metal center usually follows the trend 3d 4d 5d when the complexes are isostructural. Alkyl complexes of the first-row metals are almost always more reactive than the analogs of the third row, and they are usually more reactive than the analogs of the second row. For example, CpRu(CO)jMe requires higher temperatures to insert CO in the presence of added phosphine than does the analogous iron complex, and the osmium analogue fails to react. Similar trends have been observed for other alkyl complexes of metal-carbonyl fragments. For CpM(CO)jR complexes, the relative rates of insertion are M = Cr and Mo W, and for RM(CO)j, the relative rates are M = Mn  

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