SCIENCE

Nobel Prize Winner Henry Taube Discusses His Research

Awardee talks about the work in inorganic oxidation-reduction reactions that won him the prize in chemistry, as well as current work in his lab

Rudy M. Baum, C&EN San Francisco

"I look back on those days, and it was an exciting time for me. I did not realize how much the field would eventually develop, but I knew that I was seeing things in a way that I had not seen them only a year before. An area of chemistry was opening up for me.

"It would be hard for a student nowadays to put himself in my situa­tion because all these things are so obvious."

The speaker is Henry Taube, pro­fessor of chemistry at Stanford University, Palo Alto, Calif., and winner of the 1983 Nobel Prize in Chemistry. The field is inorganic oxidation-reduction (redox) reac­tions. And one of the major forces in making many of the details of such reactions "obvious" has been more than three decades of pioneer­ing research by Taube himself. In a recent interview, Taube discussed the research cited by the Royal Swedish Academy of Sciences in awarding him the Nobel, as well as the variety of research currently being pursued in his laboratory.

Taube did his doctoral research with chemistry professor William C. Bray at the University of Cali­fornia, Berkeley. He received a Ph.D. degree in 1940. His first ac­ademic position was at Cornell Uni­versity, where he worked to estab­lish criteria for distinguishing be­tween one- and two-electron redox processes.

It was only after he moved to the University of Chicago in 1946 that he became interested in coordina­tion chemistry as a subject for research, Taube says. One reason was that a number of his colleagues at Chicago—among them Willard F. Libby, James Franck, and Frank H. Westheimer—also were interest­ed in the subject. Another reason was that he was requested to teach an advanced inorganic chemistry course. Instead of using a standard text for the course, Taube decided to pursue a descriptive approach and drew material from the volume of the Gmelin references series which describes the chemistry of the cobaltamines.

"I became interested in why some of these compounds undergo substi­tution so slowly," Taube recalls. "I was very interested in substitution on carbon and it became obvious to me that the same issues were rele­vant to the whole field of inorganic chemistry, which had not really been looked at systematically. Be­cause a number of metal ion centers undergo substitution slowly, I real­ized that one could use ordinary techniques to follow the rates of reaction. And I began to wonder about why certain metal centers un­dergo substitution slowly and oth­ers much more rapidly."

From those beginnings, Taube de­veloped experimental methods for studying, and a theoretical frame­work for understanding, the rates of substitution of ligands on transi­tion metal ions. In a classic paper [Chem. Rev., 50, 69 (1952)], he pre­sented a correlation between ligand substitution rates and the electron­ic configuration of the transition metal ion. That correlation remains in widespread use today. "I am still rather proud of that piece of work," he says. Its primary flaw, he con­

tends, is that it was developed be­fore crystal field theory and ligand field theory became well established among chemists and, hence, the cor­relation is couched in terms of va­lence bond theory.

That work led to the research on the mechanisms of redox reactions involving transition metal ions for which he is probably best known. Because the rate of substitution on chromium(II) is much greater than on chromium(III), Taube reasoned that they would be a useful couple for studying whether what is called an "inner-sphere activated complex" is realized in redox reactions.

The first experiments were done in 1953. None of Taube's graduate students were interested in perform­ing the experiments, so Taube did them himself. "They were test tube experiments to begin with, which is something I still find interesting," he says.

In the first, simple experiment, Taube added solid iodine (I2) to an

Taube: test tube experiments to begin

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Science

aqueous solution of Cr2+ that had been prepared by then-graduate stu­dent Robert A. Plane for studies of the chromium self-exchange reac­tion. On mixing, the solution turns green and then the green color slowly fades to a color characteris­tic of Cr(H20)6

3 + . The green color is due to (H20)5CrI2+. That it fades indicates that the monoiodo species is unstable with respect to the hexaquo species and that the Cr(II)-iodine bond is formed before the chromium is oxidized.

The next step was to extend the idea to a reaction between two met­al ion complexes. Taube needed a metal complex that could oxidize Cr2+, that contained in its coordina­tion sphere something that could act as a bridging group, and that maintained its integrity in solution. Another graduate student, Ronald L. Rich, suggested using a cobalt-amine. The first test tube reaction used chloropentaaminecobalt(III) and followed the same pattern of color changes observed in the ex­periment using-I2 as the oxidizing agent. Subsequent experiments fol­lowed the reaction in a solution con­taining radioactively labeled chlo­ride ion and showed that the chloro-pentaquochromium(III) complex product did not contain radioactiv­ity. The experiments [}. Am. Chem. Soc, 26, 2103 (1954)] showed that the reaction proceeds via:

Cr2+ + (NH3)5CoCl2+ -> [(NH3)5Co.. .CI.. .Cr(H20)5]4+ — Co(H20)6

2+ + 5NH 4+ +

(H20)5CrCl2+

In the reaction, the chlorine atom acts as a bridging group between the two metal ion complexes. In such "inner-sphere" redox reactions, one can think of an atom rather than an electron being transferred.

"The experiment was important," Taube says, "because all we knew before it was that in these reactions an electron somehow finds its way through two intact coordination spheres. We didn't know anything about the distances involved or about the geometry of the activated complex. With this, we could say something definite about the mecha­nism and about distances and geom­etry." Largely as a result of Taube's

Taube: an area was opening up tor me

work, the terms inner sphere and outer sphere mechanisms are now standard in inorganic chemistry.

In subsequent research at Chica­go and Stanford, where Taube moved in 1962, Taube studied the mechanism of redox reactions in­volving what is known as "remote attack." Taube and Francis. R. Nord-meyer first demonstrated such a mechanism in 1966. The experimen­tal system used Cr2+ and Co(III) complexed with a large, conjugated, organic l igand. The researchers showed that the reduction of Co(III) proceeds via an attack by Cr2+ on the ligand rather than on Co(III) directly.

Taube, however, calls the results of that experiment somewhat disap­pointing because a mismatch of or­bital symmetries causes the reaction to proceed by a stepwise mecha­nism rather than by a process in which the electron tunnels through the ligand directly to the metal ion center.

That research, however, led Taube to his studies of rubidium chemistry. Using Ru(III) in place of Co(III) in similar experiments eliminates the mismatch of orbital symmetries [both the Ru(III) and the ligand have π orbital symmetries], and the reac­tion proceeds at a much faster rate. The reaction demonstrates remote attack with true direct transfer of the electron to the metal center through the ligand.

Taube's studies of rubidium chem­istry also established several research

directions that continue in his labo­ratory today. For instance, Ru(II), he points out, undergoes what is known as π backbonding to a de­gree unprecedented among diposi-tive metal ions. In π backbonding of Ru(II) to dinitrogen, the metal donates electrons from one of its π orbitals to the nitrogen molecule's π antibonding orbital. A systematic study of the effects of such interac­tions in complexes containing ru­bidium or osmium, which Taube de­scribes as "very significant," is a theme of current research in his laboratory.

Rubidium also played a role in Taube's development and study of mixed-valence cations: molecules containing two metal ions in differ­ent oxidation states linked by a bridging ligand. One of the first such ions—[(NH3)5Ru(pyrazine)Ru-(ΝΗ3)5]5+—was prepared and char­acterized by Carol Creutz and Taube in 1969 and is now commonly known as the Creutz-Taube ion. Such molecules allow the study of intramolecular electron transfer, thus eliminating the need to consid­er the work of bringing two reac-tants together in solution in inter-molecular electron transfers. In ad­dition to allowing the study of intramolecular electron transfer, some such ions, especially those con­taining osmium, exhibit unique properties. This is because the elec­tron derealization over the metal ions and the bridging ligand is so complete that assignment of differ­ent oxidation states to the metal ions—for example, Os(II)/Os(III) linked via pyrazine—is no longer appropriate. Instead, both ions es­sentially possess a charge of +2.5.

In the past, Taube's research has at times been characterized as some­what esoteric. As is so often the case in pure basic research, however, it has just taken a while for other scientists to apply the results to oth­er problems. Taube points to recent independent research by Harry B. Gray at California Institute of Tech­nology and Stephan S. Isied at Rutgers University as a case in point. Both are using rubidium to form what amounts to a mixed-valence cation out of the cytochrome C pro­tein molecule to study the electron transfer process in that protein. D

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