C&EN 80th anniversary
C&EN | Periodic Table



uring the 1950s, we demonstrated that several transition-metal ions, among them rhodium(III), can activate H2 homogeneously in solution by a previously unrecognized route, namely electrophilic attack, leading to heterolytic splitting, as exemplified by the pictured catalytic cycle [Canad. J. Chem., 37, 1933 (1959)]. These studies also accorded among the earliest recognition to the roles of transition-metal hydrides as catalytic intermediates.

The remarkable renaissance of transition-metal coordination and organometallic chemistry during the intervening half century has been driven in considerable measure by the highly novel, diverse, and useful catalytic applications of this chemistry. Rhodium catalysts have been prominently represented among such applications, exhibiting a scope and versatility that probably are unmatched by those of any other element. Among the rhodium-catalyzed reactions that have received significant attention are the hydrogenation of olefins, including the first commercial asymmetric catalytic process (L-dopa synthesis); hydrogenation of arenes; hydroformylation of olefins; olefin-diene codimerization; and carbonylation of methanol to acetic acid.

In a 1966 C&EN article (Oct. 31, page 68), I called attention to a pattern of striking parallels (subsequently embodied in the "isolobal analogy" concept) between the reactivity patterns of certain low-spin transition-metal complexes and those of reactive organic species of related configurations; for example, d6 complexes (e.g., RhIII) and carbonium ions; d7 complexes (e.g., RhII) and free radicals; or d8 complexes (e.g., RhI) and carbenes or carbanions. The versatility of transition-metal complexes as catalysts and catalytic intermediates can be attributed in considerable measure to the accessibility of one or more of these reactive configurations, and in the case of rhodium to all of them, as manifested in the following prototypical reactions: RhIII (d6) + RH Rh–R + H+ (R = H or aryl); RhII (d7) + RBr Rh–Br + R· (R = benzyl); 2RhII (d7) (or [RhII]2) + RH Rh–R + Rh–H (R = H or CH3); RhI (d8) + H2 (or CH3I) RhH2 [or Rh(CH3)I]; RhI + CH3I Rh–CH3 + I. Combinations of these elementary steps, together with certain other characteristic reactions, notably the insertion of olefins into metal-hydrogen bonds (Rh–H + C=C Rh–C–C–H) and of carbon monoxide into metal-alkyl bonds (Rh–R + CO Rh–C(=O)R), account for much of the rich and diverse catalytic chemistry of the transition metals, particularly of the later ones.

My own research interests have focused particularly on the mechanistic aspects of homogeneous catalysis by transition metals. Rhodium has played a central role in these endeavors. While most transition-metal-catalyzed reactions proceed through combinations of steps such as those identified above, rhodium-catalyzed reactions are distinctive in that the relative rates of the successive steps and stabilities of the diverse reaction intermediates often are matched to a degree that enables all or most of them to be directly observed and characterized. Examples of rhodium-catalyzed reactions whose mechanisms have been elucidated in impressive detail, with "interception" and characterization of most of the intermediates, include the RhCl(PPh3)3-catalyzed hydrogenation of olefins (Wilkinson's catalyst), the Rh(diphosphine)-catalyzed asymmetric hydrogenation of enamides (Knowles' catalyst), and the [Rh(CO)2I2]-catalyzed carbonylation of methanol (Monsanto acetic acid process).

Rhodium complexes, notably porphyrins, also exhibit distinctive patterns of reactivity toward C–H bonds. As with other electrophilic metal ions, aromatic C–H bonds are cleaved heterolytically by Rh(IIII) to form Rh-aryl adducts. On the other hand, in reactions that have few parallels for other metals, selected benzylic and aliphatic C–H bonds are cleaved homolytically by Rh(II) to form the corresponding rhodium benzyl or alkyl adducts, along with rhodium hydrides; e.g., 2RhII (or [RhII]2) + CH4 Rh–CH3 + Rh–H. These reactions have thus far been demonstrated to occur stoichiometrically. The challenge of incorporating them into useful catalytic cycles for the functionalization of hydrocarbons remains to be addressed.

The modern phase of research on the coordination and catalytic chemistry of rhodium extends back about half a century. I have been fortunate to have been involved, directly or indirectly, in the earliest stages of this research and in many of the intervening developments. The field continues to show impressive vitality and to yield novel discoveries and important insights, as attested to by some of the relatively recent developments in rhodium porphyrin chemistry. I anticipate that this will continue to be the case for some time to come.


Jack Halpern is the Louis Block Distinguished Service Professor of Chemistry Emeritus at the University of Chicago. Following Ph.D. studies at McGill University, he served on the faculty of the University of British Columbia until 1962, when he moved to the University of Chicago.


Chemical & Engineering News
Copyright © 2003 American Chemical Society

Name: From the Greek rhodon, rose.
Atomic mass: 102.91.
History: Discovered in 1803 by English chemist William H. Wollaston.
Occurrence: Found with nickel and copper deposits in Canada. Very few rhodium minerals exist.
Appearance: Silvery metal.
Behavior: Rhodium compounds stain the skin and can be highly toxic and carcinogenic.
Uses: Primarily used as an alloying agent to harden platinum and palladium. Such alloys are used in furnace windings, thermocouple elements, bushings for glass-fiber production, electrodes for aircraft spark plugs, and laboratory crucibles. Rhodium is also used as an electrical contact material, in jewelry, and as a catalyst. Plated rhodium, produced by electroplating or evaporation, is exceptionally hard.

E-mail this article to a friend
Print this article
E-mail the editor

C&EN | Periodic Table | How To Reach Us | How to Advertise | Editorial Calendar | Email Webmaster

Chemical & Engineering News
Copyright © 2003 American Chemical Society. All rights reserved.
• (202) 872-4600 • (800) 227-5558

CASChemPortChemCenterPubs Page