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

The linear "compensation plot" of Delta H double dagger versus Delta S doub le dagger for associative substitution reactions of Ru5C(CO)(14) {P(OPh3)} with seven P-donor nucleophiles suggests that an isokinetic temperature, T- iso of 253 +/- 10 K exists. A detailed statistical analysis by the Linert-E xner method shows that the data are consistent with there being a genuine i sokinetic temperature at 245 K at which reactions with six out of the seven nucleophiles proceed at the same rate. This can be shown more easily, grap hically more vividly, and with quantitatively the same or better results, b y a simplified version of an earlier method due to Krug, Greiger et al. in which Delta H double dagger Values are found to depend linearly on correspo nding values of Delta G double dagger calculated at a suitably chosen tempe rature. This isokinetic behaviour is closely related to that shown by the l inear free energy analysis of the rates in terms of the electronic and ster ic properties of the nucleophiles. The temperature dependence of the sensit ivity of the rates to these electronic and steric properties suggests that the major factors involved are entropic rather than enthalpic, reactions wi th larger nucleophiles actually being favoured by enthalpic factors. Steric profiles obtained at different temperatures all pass through a common poin t with an isokinetic cone angle of 153 degrees. A few examples of other rea ctions of organometallic or coordination compounds that show linear compens ation plots of Delta H double dagger versus Delta S double dagger are also analysed by the Linert-Exner and Krug-Greiger methods. Some do show unambig uous isokinetic behaviour but others do not, even though the compensation p lots appear to be linear.