General discussion

Abstract

Newcastle upon Tyne) said: With reference to the paper by Bond et al., in the dihydrido-rhodium phenylphosphine complex the appreciably different molecular environment of the two hydrido-ligands should lead to an appreciable difference in relative reactivity. Why do the authors therefore consider the first hydrogen to be transferred to be that regarded as the nearer to the co-ordinated olefin ? Does this not give undue weight to the unascertained importance of steric probability of reaction and neglect the importance of the energetics of hydrogen transfer ?The authors also rationalize the effect of change in the halide ligand on olefin isomerization and hydrogenation in steric terms. Our studies, reported in the following paper, reveal a comparable effect in a series of complexes where the halide ligand is relatively remote, and the effects of the iodo-ligand and the much smaller -CN group are similar. Why do the authors not take account of the undoubted change in electronic character of the ligands Cl-, Br, I-, in considering their results ? Dr. G. C. Bond (Johnson Matthey and Co. Ltd.) said: McQuillin implies, I believe, the two hydrogen atoms on the rhodium complex to be equivalent; this is not so. My mechanism is based on the hydrogen atom trans to the halogen being the more reactive on the grounds that it is closer to an sp2 carbon atom than the other. Concerning the manner in which the nature of the halogen atom affects the course of the reaction, McQuillin may be correct in supposing that steric effects are not alone responsible for the observed differences : but I find it difficult to know how to distinguish between electronic effects transmitted through the metal atom from steric effects which alter the energy of the transition state. Newcastle upon Tyne) said : My point is not that the two hydrogen atoms in the rhodium complex are equivalent, but that they are indeed non-equivalent in their molecular environment, and hence in their reactivity. Dr. F. J. McQuillin (University of