Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

RHODIUM AT A GLANCE

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.

IRIDIUM

RICHARD EISENBERG , UNIVERSITY OF ROCHESTER

The lab was dark--intentionally so--except for the eerie purplish glow of a handheld dark light and the bright red emis-sion of an unlabeled sample vial that belonged to a student no longer there. The year was 1980, and I had just heard a stimulating lecture from Allen J. Bard about electrochemiluminescence using Ru(bpy)₃²⁺ and oxalate. When asked what to look for in trying to extend this to other systems, he said, "First, find a good luminescent complex."

Hence, the search--a poor man's high-throughput screening done by opening up every sample drawer in the lab and walking through the darkened lab with a handheld UV light. The red light was impressive, but its origin was a mystery. However, within several days, the compound was identified as an iridium(I) carbonyl complex with triphenylphosphine and the anionic ligand maleonitriledithiolate (mnt). Derivatives with other phosphines, phosphites, and even isocyanides followed quickly, most showing a distinctive red luminescence that was tunable by changing the ligand.

I have been fascinated by iridium since I first learned of the complex reported by Lauri Vaska that bears his name, IrCl(CO)(PPh₃)₂, nearly 40 years ago. This remarkable compound forms adducts with small molecules like O₂ and SO₂ and activates others like H₂, methyl iodide, and silanes by oxidative addition. The importance of this chemistry, which is described in every modern inorganic and organometallic chemistry textbook, relates to the controlled breaking of bonds for substrate activation in catalysis. The key to this process for Vaska's complex and its derivatives comes from the aesthetically pleasing electronic structure of square-planar complexes and the diversity of its frontier orbitals--a vacant p orbital and filled d orbitals for and synergic bonding with a substrate and a filled d_{z 2} orbital that can serve as an electron-pair donor to an addend.

Not all square-planar complexes can do oxidative addition chemistry, but iridium(I) complexes like IrCl(CO)(PPh₃)₂ are particularly adept at it. As in the study of any chemistry, the more one looks, the more there is to ask. For oxidative addition to square-planar Ir(I) systems, the reaction's stereoselectivity is a question that we have probed with H₂ as substrate, using, in part, sensitive nuclear magnetic resonance methods based on parahydrogen.

Iridium(I) complexes have also been in the forefront of research addressing one of chemistry's holy grails: the activation of stable, unactivated carbon-hydrogen bonds. The seminal studies of Robert G. Bergman and William A. G. Graham 20 years ago featured iridium complexes containing the pentamethylcyclopentadienyl ligand and were followed with investigations using other Ir(I) systems having electronically related tridentate ligands. Different lines of investigation on the same problem, first by Robert H. Crabtree and more recently by Craig M. Jensen, William C. Kaska, and Alan S. Goldman, led to success with complexes having different ligands and geometries, but all unified in containing iridium. Iridium complexes are generally not as good as rhodium analogs for homogeneous catalysis because they form more stable oxidative addition products, but the Cativa system for acetic acid synthesis developed by BP employs Ir(I) and Ir(III) carbonyl iodides.

Numerous variations of Vaska's complex have been made--different halides, different phosphines, substitution of CO, and change from trans phosphines to cis--and all exhibit to differing extent the extraordinary oxidative addition chemistry shown by IrCl(CO)(PPh₃)₂. The mysterious complex that luminesced red was first synthesized as an anionic derivative, but its beautiful photoemission moved us in another direction. Again, iridium did not disappoint. Coordination of mnt to Ir(I) introduced a charge-transfer excited state, and variation of the other ligands in the complex led to subtle tuning of the emission energy.

The luminescence properties of other iridium systems have garnered great attention. Ru(bpy)₃²⁺ is arguably the most extensively studied metal complex luminophore. Yet the "isoelectronic" Ir(III) system made by Richard J. Watts containing orthometallated phenylpyridine as the chelate has been found by Stephen R. Forrest and Mark E. Thompson to be a highly efficient emitter of green light in prototypes of flat-panel displays based on electroluminescence. This phenomenon serves as the basis of OLEDs (organic light-emitting diodes), but here the iridium plays a key role in giving emission from a triplet excited state that leads to greatly increased efficiency. Extensive work has shown that the emission color can be tuned by ligand variation or substitution, and studies suggest that these Ir(III) systems may have important applications in emerging display technologies.

From oxidative addition, bond activation, and catalysis to electronic structure and luminescence, the allure of iridium is powerful and seductive. Ensconced between osmium and platinum, the element's compounds possess properties and reactivity that continue to draw me to the joys of iridium.

Richard Eisenberg is the Tracy Harris Professor of Chemistry at the University of Rochester. He is editor-in-chief of Inorganic Chemistry and is the 2003 recipient of the ACS Award for Distinguished Service in the Advancement of Inorganic Chemistry.

IRIDIUM AT A GLANCE

Name: From the Latin iris, rainbow. Iridium salts are highly colored.

Atomic Mass: 192.217.

History: Discovered with osmium by English chemist Smithson Tennant in 1803 in the residue left when crude platinum is dissolved by aqua regia.

Occurrence: Found in platinum ores and as a by-product of mining nickel.

Appearance: Silvery white metal.

Behavior: The pure metal is very brittle and difficult to machine, but it is also the most corrosion-resistant metal known.

Uses: Primarily used as a hardening agent for platinum. It is also used in helicopter spark plugs.

MEITNERIUM

PATRICIA RIFE, UNIVERSITY OF MARYLAND

It is quite fitting that Austrian Lise Meitner, a physicist and pioneer in Max Planck's quantum circle in Berlin, would have such a rare, short-lived element named after her. She was long-lived herself (1878-1968) and nominated four times for the Nobel Prize for her interpretation of nuclear fission in 1938, after a traumatic escape from Nazi Germany -- despite the fact that her research partner for over 30 years, Otto Hahn, was awarded the 1946 Nobel Prize in Chemistry for the discovery of fission.

After a dramatic escape from Nazi Germany in 1938, orchestrated by Niels Bohr in Copenhagen and many colleagues worldwide, Meitner worked through the war years in the Stockholm-based Nobel Institute for Physics.

It was a fateful letter from Hahn asking her to explain his finding of barium when uranium was bombarded with neutrons that led to her famous interpretation of nuclear fission. Nazi officials had been notified not to let the Jewish woman scientist out of Berlin, but alone with the 34-year-old nephew Otto Robert Frisch over the holidays, Meitner had the insight to recognize that a tremendous amount of energy would be released in this new process, nuclear fission. They wrote up their findings for Nature, which were published in February of 1939 and confirmed by Frisch at Bohr's Institute.

When Bohr made the announcement of fission to colleagues in America in January 1939, John A. Wheeler "let the cat out of the bag" by announcing it to a student group in Princeton. However, their March 1939 "Mechanism of Nuclear Fission" paper in Nature gave full credit to the teams of Meitner and Frisch, as well as Hahn and Strassmann.

Meitner

Hence, most chemists and physicists were shocked after World War II that she did not receive her share of the credit by sharing the 1946 Nobel Prize in Chemistry with Hahn, though Meitner was duly lauded in America. She was interviewed by Eleanor Roosevelt on NBC Radio immediately after the bombing of Nagasaki and arrived for her first trip to the U.S. in 1946, where she was named "Woman of the Year" by the U.S. Women's Press Club. She dined with Harry Truman in the White House (January 1946); was awarded more than 10 honorary doctorate degrees; and, by the end of her life, had received the prestigious Order of the Pour le Mérite Award (shared with Hahn), presented by the president of Germany in 1961, and the Enrico Fermi Award.

To her credit, Meitner was the first woman in all of Austria to earn a Ph.D. in physics (under the esteemed Ludwig Boltzmann and Stefan Meyer) and hence participated in the fierce debates about the "reality" of the atom before moving to Berlin in 1907. There, working first as a postdoc assistant to Max Planck, she spent long hours in a refurbished carpenter's shop outside of the Institute for Chemistry, counting -decay chains on a "scintillating screen" while Hahn worked out the complex chemistry relating to their joint field of "radioactivity." His mentor, Ernest Rutherford, would send them packages in the mail, and Meitner was known to startle the postman when her Geiger counter (made by their friend Hans Geiger) would go off before he announced that a parcel had arrived from Cambridge, England. Meitner also spent time playing concert piano while Albert Einstein played his violin during friendly evenings at Planck's home, and she was a dear friend of Max von Laue, Paul Ehrenfest, Geiger, and other physicists and chemists in their small, intimate circle of Berlin scientists.

Meitner volunteered in one of the first primitive mobile X-ray units during World War I. After the war, she returned from her Austrian family to remain in Berlin until Hitler's rise to power drove many of her colleagues away. Hahn became the director of the Kaiser Wilhelm Institute for Chemistry, and Planck was the president of the KW Gesellschaft, so they protected Meitner until 1938, when the racist policies of the Third Reich drove her, at age 65, to finally escape from Germany.

Meitner's contributions to both science--more than 120 articles published on radioactive substances and their properties--and society are long lasting. As this pioneer said in Cambridge, England, at the end of her illustrious career: "I believe young people think about how they would like their lives to develop; when I did so, I always arrived at the conclusion that life need not be easy, provided only that it is not empty."

Patricia Rife is a historian of science whose book "Lise Meitner and the Dawn of the Nuclear Age" is being developed into a screenplay for a film. Rife is a professor in the Graduate School of E-Commerce at the University of Maryland, and she studies the sociocultural impact of science and scientists upon modern societies.

MEITNERIUM AT A GLANCE

Name: Named for Lise Meitner, the Austrian physicist who first suggested a theory of spontaneous nuclear fission.

Atomic mass: (268).

History: First synthesized in 1982 by Peter Armbruster, Gottfried Münzenberg, and coworkers at the Gesellschaft für Schwerionenforschung, in Darmstadt, Germany, by bombarding bismuth-209 with accelerated iron-58 nuclei.

Occurrence: Artificially produced. Only a few atoms of meitnerium have ever been made.

Appearance: Metal of unknown color.

Behavior: Highly radioactive.

Uses: No commercial uses.

NICKEL

DAVID HANSON, C&EN WASHINGTON

If there ever was a utilitarian metal, it's nickel. This well-known transition element may have more varied applications than any other metal. It is used in everything from our coins to our automobiles and from jewelry to paper clips, and new uses are found all the time.

Basically, nickel is a hard, malleable, ductile, lustrous, silver-white metal that takes a high polish. It conducts heat and electricity and is slightly magnetic. It forms numerous compounds, many of them blue or green, and finely divided nickel can adsorb hydrogen.

But it is as an alloy with other metals that nickel really shines. The first reported use of nickel was in a nickel-copper-zinc alloy produced in China in the Middle Ages. It is believed that some alloys were produced in prehistoric times. Today, an estimated 85% of nickel ends up as alloys.

The largest use is in making stainless steel. As much as 70% of nickel goes to make stainless or other steel alloys. With concentrations of up to 45%, nickel adds strength and corrosion resistance. Surprisingly, 16% of stainless steel goes into the chemical process industry. Electronics consume 18%; auto manufacturing, 15%; and the food and beverage industry, 13%.

In addition to its use in steel alloys, nickel forms useful alloys with other metals. Copper-nickel alloys offer a good compromise between strength and ductility and resist corrosion in saltwater, nonoxidizing acids, and alkalies. These alloys are used in industrial plumbing and petrochemical equipment.

Nickel-copper is also the alloy of which coins are made. The U.S. nickel is 25% nickel and 75% copper.

Other useful alloys include nickel-chromium and nickel-molybdenum combinations that are the basis for materials that can withstand extremely corrosive chemical plant environments, such as hot sulfuric and phosphoric acids, hydrogen chloride gas, and other oxidative conditions.

Electroplating is the second largest use for this versatile metal. The process is used to produce corrosion-resistant and decorative finishes, as well as substrates for chromium coatings. Nickel can be plated on many surfaces, including plastics. Automobile trim, bathroom fittings, and electronic connectors are just a few of the many applications.

There is also a process for plating nickel without an electric current. This "electroless" process makes very uniform plating. Other materials can be added to improve the finish, such as Teflon to increase lubricity or silicon carbide for wear resistance. This process is used on computer hard drives for a smooth, nonmagnetic base for the magnetic recording layer.

Nickel also happens to be an excellent catalyst for many chemical reactions. By itself or combined with other metals, nickel is used for a myriad of industrial and research applications. The most famous nickel catalyst is called Raney nickel. Developed by Murray Raney in the 1920s, it is 90% nickel and 10% aluminum.

All of these uses demand a lot of nickel. The U.S. consumes more than 195,000 metric tons of nickel yearly. But the last nickel mine in the U.S. closed in 1987. Most new nickel comes from Canada and Australia. The two most common ores are nickel-iron-sulfide pentlandite, (Ni,Fe)₉S₁₆, and a nickel silicate contained in hydrated magnesium, usually garnierite, (Ni,Mg)₆ Si₄O₁₀(OH)₈.

But at a cost of $8,000 per ton, nickel is not cheap. So there is an efficient recycling system to recover and reuse nickel. More than 110,000 tons of nickel were recovered from scrap in the U.S. last year, about 57% of total consumption, according to the U.S. Geological Survey.

FIREBALL A 3-mm droplet of nickel-zirconium, heated to incandescence, hovers between electrically charged plates inside the Electrostatic Levitator at NASA's Marshall Space Flight Center in Huntsville, Ala.