Isokinetic relationships (IKRs) in organometallic and coordination chemistry: Reactions of Ru5C(CO)14{P(OPh)3} and other complexes

The linear compensation plot of H versus S for associative substitution reactions of Ru5C(CO)14 {P(OPh3)} with seven P-donor nucleophiles suggests that an isokinetic temperature, Tiso, of 253±10K exists. A detailed statistical analysis by the Linert-Exner method shows that the data are consistent with there being a genuine isokinetic temperature at 245K at which reactions with six out of the seven nucleophiles proceed at the same rate. This can be shown more easily, graphically more vividly, and with quantitatively the same or better results, by a simplified version of an earlier method due to Krug, Greiger et al. in which H values are found to depend linearly on corresponding values of G calculated at a suitably chosen temperature. This isokinetic behaviour is closely related to that shown by the linear free energy analysis of the rates in terms of the electronic and steric properties of the nucleophiles. The temperature dependence of the sensitivity of the rates to these electronic and steric properties suggests that the major factors involved are entropic rather than enthalpic, reactions with larger nucleophiles actually being favoured by enthalpic factors. Steric profiles obtained at different temperatures all pass through a common point with an isokinetic cone angle of 153°. A few examples of other reactions of organometallic or coordination compounds that show linear compensation plots of H versus S are also analysed by the Linert- Exner and Krug-Greiger methods. Some do show unambiguous isokinetic behaviour but others do not, even though the compensation plots appear to be linear.