When mixtures of unsaturated substances, or compounds containing multiple sites of unsaturation are hydrogenated using RhCI(PPh& as catalyst, it is possible that the substrate which reduces slowest as a single substrate can be hydrogenated preferentially in the mixture. The degree of selectivity achieved can be enhanced by the addition of polar solvents, the effects of which are discussed. The degree of participation of the two kinetically indistinguishable forward routes, Le., via substrate attack on a hydrido intermediate, or hydrogen attack on a substrate intermediate, may aho be of importance in determining the degree of selectivity achieved.* The homogeneous hydrogenation ability of RhCl(PPh& was discovered independently (1964)

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