Energy bond dissociation, Group 14 hydrides

Table 4.55. First M—H bond dissociation energies ATTmiki) for group 4, 6, and 10 metal hydrides...

We have noted in our discussion of previous reaction classes that relative rates can be correlated with bond dissociation energies. The same is true of migratory insertions. As shown in the reactions grouped under Eq. 12.44, migratory insertions of CO or alkenes into M-alkyl bonds are typically much faster than insertions into M-hydride bonds. This is due to the much stronger M-H bonds relative to M-R bonds. 

Fig. 28 Nonrelativistic and relativistic bond lengths, (see also Table 13 for various values for RgH), dissociation energies, >e, and force constants, k, of the group-11 hydrides. Reprinted with permission from [194]. Copyright 1996 Elsevier...

Besides these determinations of static structures, further studies of the kinetics of oxidative addition to Ir(I) complexes have been made. Beginning with Ir(CO)H(PPh3)3 (VIII), the first step is seen as dissociation to the four-coordinate planar intermediate (IX). This then deforms and undergoes concerted cis addition to yield the adduct (V) (179,224). In a comparison of group IV hydrides, PhsMH, the rates increased in the order Sibond energy but to the effects of differing polarizability and solvation of M-Ir products on the activation energy for addition. 

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