Ab initio calculations with effective core potential have been used to study the mechanism of substitution reactions of 17-electron and 18-electron (e-) transition-metal hexacarbonyl complexes. Pseudo-C2v transition states were found for the substitution reactions investigated in this paper. The significant difference in the substitution reaction rates between 17-e- and 18-e- hexacarbonyl complexes is attributed to the significant difference in the valence-electron charge distributions of the corresponding transition states. The Laplacian of the valence-electron density indicates that the single-electron difference between the 19-e- and 20-e- transition states leads to a significant difference in the valence-electron charge concentrations. The angle between the two charge concentrations which face the entering and leaving ligands for a 20-e- transition state is much smaller than that for a 19-e- one. The more open coordination site of the 19-e- system allows more effective bonding of the entering/leaving ligands to the central metal atom simultaneously. Therefore, the transition state is much more stable for the substitution reactions of 17-e- transition-metal complexes.
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Lin et al. (1992) studied this question.