Kinetics of reactions of Os6(CO)18 with some P-donor nucleophiles: Some unusual isokinetic relationships

The cluster Os₆(CO)₁₈ with its bi-capped tetrahedral Os₆ core reacts with some P-donor nucleophiles in toluene via associative paths, but the range of P-donors that can react at convenient rates for kinetic study is limited by the exceptional dependence of the rates on the basicity and size of the nucleophiles. The observed linear free energy relationship (LFER), shown by the small set of accessible data is accompanied by the existence of an isokinetic temperature of 89.5 ± 3.5 °C that controls the effect of nucleophiles basicity. This isokinetic behaviour is known to be an inevitable logical consequence of the existence of the LFERs at different temperatures but other necessary isokinetic properties are also demonstrated to exist. These are an isokinetic basicity (IKB) parameter, which results in the predicted temperature independence of rates of reaction with nucleophiles of a certain basicity, and an isokinetic cone angle parameter (IKCA) that is predicted to result in the temperature independence of the rates at a certain nucleophile cone angle.