Experimental evidence in octahedral substitution

We will now consider additional evidence used to support mechanistic arguments. 

FIGURE 12.4 Change in energy levels (in terms of upon ligand dissociation from octahedral MLgto square-pyramidal MLj.  

System Octahedral LFSE Square-Pyramidal LFSE EEAE Octahedral LFSE Square-Pyramidal LFSE LFAE 

For a square-pyramidal transition state, LFAE = square pyramid LFSE - Octahedral LFSE, for j donor only. 

Electronic and steric factors also influence substitution reaction rates of octahedral complexes. The inequalities below indicate relative rates for ligand exchange via presumed dissociative mechanisms. 

Ligand Field Activation Energies Calculated by Angular Overlap 

FIGURE 12-3 Activation Energies and Reaction Enthalpies, (a), (b), Large E, slow reaction, (c) Small Ea, fast reaction, (a), (b), hH 0, large equilibrium constant  

Even for thermodynamically favorable reactions, a large activation energy means that the reaction will be slow. For thermodynamically unfavorable reactions, even a fast reaction (with small activation energy) would be unlikely to occur. The rate of reaction depends on the activation energy, as in the Arrhenius equation 

FIGURE 12-4 Stabilization Energy and Experimental Half-Lives for Water Exchange. (Angular overlap data (solid line) from J. K. Burdett, J. Chem. Soc. Dalton, 1976, 1725. Half-lives for water exchange (dashed line) from F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, 2nd ed., John Wiley Sons, 

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The mechanism may be associative (A or f) or dissociative (D or f), and it is not at all easy to distinguish between these, even though the rate laws are different. An associative mechanism involves a 7-coordinate intermediate or transition state and, sterically, an associative pathway seems less likely than a dissociative one. Nevertheless, activation volumes do sometimes indicate an associative mechanism (see Table 25.4). However, for most ligand substitutions in octahedral complexes, experimental evidence supports dissociative pathways. Two limiting cases are often observed for general reaction 25.22 ... 

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