Ligand substitution in octahedral complexes

Qualitative information about the rate of ligand substitution in octahedral complexes is summarized in Table 18.1. Quantitative data are obtained by application of ligand field theory. The rate constants may vary by many orders of magnitude. Thus, for the exchange of water molecules in the first coordination sphere of 3d transition elements the following values have been measured Cr (d" ) 7 10 s Cr (d ) 5-10- s-i Mn2+(d5) 3-lO s- Fe2+(d ) 3-10 - Co + d ) 1 lOS Ni2+(d ) 3 104s-i. 

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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