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

Unfortunately, nickel comes with an evil side. Several nickel compounds are known human carcinogens. Nickel refiners had a number of health problems in the past, but current exposures to nickel in the workplace are much lower. Still, caution is taken with nickel refinery dust and especially nickel subsulfide (Ni₃S₂). Another compound of concern is nickel carbonyl, a highly toxic, volatile liquid used to purify nickel or to produce fine nickel particles. U.S. and international health agencies have set exposure standards for these and other nickel compounds.

Another health issue is contact dermatitis from exposure to nickel. Reactions to nickel alloys in earrings used for pierced ears are the most frequent, but itchy rashes can occur on any body part that comes into prolonged contact with nickel. The European Union has banned earrings with more than 0.05% nickel and some nickel-plated jewelry. The American Academy of Dermatology says that nickel allergies are the most common chemical allergy causing skin problems.

Nickel use continues to grow as new applications are found. Nanotechnology, electronics, and catalysis are areas of exciting nickel research. Use of the metal is rising each year, and the industry is confident about its future. This is one element where you don't have to exaggerate when you say that you're getting your nickel's worth.

David J. Hanson is assistant managing editor for government and policy at C&EN. He has worked for the magazine since 1977.

NICKEL AT A GLANCE

Name: From the German kupfernickel, loosely meaning false copper.

Atomic mass: 58.69.

History: Discovered in 1751 by Swedish chemist Axel F. Cronstedt in niccolite.

Occurrence: Rare in Earth's crust, but many experts believe it is far more common in its molten core.

Appearance: Silvery white metal.

Behavior: Resistant to oxidation. Many forms of nickel are harmful to humans.

Uses: Widely used to make stainless steel and many other corrosion-resistant alloys. Also used in rechargeable batteries and heating elements. The coin contains about 25% nickel.

PALLADIUM

LARRY OVERMAN, UNIVERSITY OF CALIFORNIA, IRVINE

The Englishman William Hyde Wollaston discovered palladium in 1803 in South African crude platinum ore. Palladium derives from the Greek name Pallas, a name associated in Greek mythology with Athena, the goddess of wisdom. Apparently, Wollaston was stimulated to use this name by the asteroid Pallas, one of the brightest of asteroids, which was discovered at about the same time.

Palladium is a silver-white metal that does not corrode in air even at high temperatures. It is the least dense of the group 10 metals, becoming soft and ductile when annealed. Palladium has been called the "amazing soaking sponge" for its ability at room temperature to absorb up to 900 times its own volume of hydrogen, a property that can be exploited to purify hydrogen or activate it for chemical reaction.

Although automobile catalytic converters use less than one-fifth of an ounce of palladium each, the largest commercial use of this metal (60% of demand in 1999, according to the United Nations Conference on Trade & Development) is in the auto industry for reducing exhaust emissions of hydrocarbons, carbon monoxide, and oxides of nitrogen. Palladium's high resistance to corrosion leads to it being employed in the electronics sector and in the formulation of dental alloys, uses that constituted approximately one-third of world demand in 1999. In the former sector, palladium is used in the production of multilayer ceramic capacitors, components of mobile telephones, personal computers, and numerous auto and home electronics. Palladium has long been used in jewelry, either alone or mixed with gold to produce "white gold."

Palladium's remarkable catalytic properties are responsible for the widespread use of this metal in the chemical industry. Palladium is employed in the production of bulk chemicals such as purified terephthalic acid (a component of artificial fibers) and nitric acid for use in fertilizers. In the specialty chemicals area, palladium catalysis has had a remarkable impact, fundamentally changing during the past 30 years how exploratory drug candidates are synthesized in the pharmaceutical and biotechnology industries and to some extent how fine chemicals and pharmaceuticals are manufactured.

Finely divided palladium has long been employed in catalytic hydrogenations, where molecular hydrogen is added across carbon-carbon or carbon-hetero multiple bonds. The modern era of organopalladium chemistry can be traced to 1960, when the Wacker process for producing acetaldehyde by air oxidation of ethylene using a PdCl₂/CuCl₂ catalyst was introduced. Five years later, it was shown that allyl and alkene complexes of PdCl₂ react with carbon nucleophiles to form new C–C s-bonds. In subsequent years, the use of palladium catalysis to fashion C–C bonds has become a staple in the synthesis of fine chemicals. Of particular importance are cross-coupling reactions in which two organic fragments are reliably joined by C–C bond formation. I venture to guess that it would be a rare month (likely week) when a medicinal chemist involved in discovery research does not employ some palladium-catalyzed coupling reaction.

Many other reactions of organic compounds are catalyzed by palladium, such as carbonylations, hydrosilylations, and molecular rearrangements, the latter reaction first attracting my research group to this area in the late 1970s. The use of palladium catalysis in the synthesis of fine chemicals is certain to continue to grow, stimulated by ongoing developments such as carbon-heteroatom cross-coupling reactions and broadly useful asymmetric processes for fashioning carbon stereocenters of chiral molecules in a single configuration.

Larry Overman is a distinguished professor of chemistry at the University of California, Irvine. He is the 2003 recipient of the ACS Arthur C. Cope Award. He trained as a chemist at the beginning of the modern era of palladium chemistry, starting graduate school at the University of Wisconsin, Madison, in the same year, 1965, that palladium-promoted formation of carbon-carbon bonds was first described.

PALLADIUM AT A GLANCE

Name: Named for the asteroid Pallas.

Atomic mass: 106.42.

History: Discovered in 1803 by English chemist William Hyde Wollaston while analyzing samples of platinum ore.

Occurrence: Rare. Palladium tends to appear with deposits of platinum, nickel, copper, silver, and gold.

Appearance: Silvery white, soft metal.

Behavior: Highly resistant to corrosion.

Uses: Used as a catalyst in several industrial processes and in catalytic converters. Palladium is also used in electrical contacts, dental crowns, and jewelry.

PLATINUM

RICHARD M. GROSS, DOW CHEMICAL

When my mother's silvery white wedding ring slid off and rolled under the porch of the summer cottage in New Hampshire, I was the only one small enough to crawl beneath the porch to retrieve it. I had no idea I would, later in life, consider the material used to make that ring--platinum--one of the elements important to one of my life's pursuits.

After I found the ring, I can distinctly remember feeling sorry for my mother because it wasn't gold, like other mothers wore. I asked her about that, and I remember her answer. "This is rarer than gold," she recounted, "and it's sturdier, which means I can wear it my entire life."

So, my vast knowledge of platinum at that time was that it was used to make jewelry.

RINGING ENDORSEMENT Though it has many commercial uses, platinum is perhaps best known for its presence in jewelry.

By the time I was in high school and developing a growing interest in science, I was reintroduced to platinum while learning about thermocouples. It struck me then that this was the same material as my mother's wedding ring. How interesting that there were such significantly different uses of the element.

In college, I discovered platinum was also used as the basis of important catalysts in industry--for the production of products such as sulfuric acid, nitric acid, and hydrogen cyanide.

Some of the most interesting things I learned about this magic metal were that 90% of all platinum comes from South Africa and Russia, it's extraordinarily rare, and more than 10 tons of ore has to be mined to produce a single ounce of platinum. That's twice as much ore required to produce an ounce of gold.

Early in my career, I remember learning about the development of the Platforming (platinum and reforming) process by United Oil Products (UOP) in the late 1940s. The Platforming process is one of the few breakthrough technologies that radically changed the face of the petroleum industry and helped meet the demands of the automobile industry for higher octane gasoline.

In recent years, through Dow's 50% ownership of UOP with Honeywell, I have enjoyed learning more about the details of Platforming research. For example, it was interesting to learn that the cost of this noble metal almost killed the research project before it was started and clearly drove the development of promoters to achieve great performance at low-platinum loadings.

Platinum history takes us back 3,000 years to ancient Egypt, where metalsmiths were skilled in working with this rare metal. A 2,500-year-old coffin of an Egyptian high priestess was found with the coffin's platinum-engraved hieroglyphs still polished and lustrous.

The Incas created adornments from platinum but when they were invaded by the Spanish conquistadors, platinum was declared the "solver of less value," and the conquistadors dumped great amounts of platinum into the sea because they were fearful it would become a cheap imitation of silver.

Finally, during the 18th century, platinum's value as a metal suited for jewelry took hold, and in the 19th century, it became the standard mounting for the newly discovered gemstone, the diamond. The most famous of these diamonds--the Hope, the Jonker, and the Koh-I-Nor--are all set in platinum.

However, the largest market for platinum today is in automobile catalytic converters.

Once again, platinum is meeting the demands of society for cleaner, more environmentally friendly automobiles. Catalytic converters and, thus, platinum, have significantly increased air quality in recent decades.

Getting back to the beginning, platinum remains equally as important to the jewelry market as it is to the industrial market. So, while I've learned lots about platinum since I went searching for my mother's wedding ring, it appears as though my early lessons remain true to the element's role in our lives. And as a testament to this material, my mother continues to wear her ring, 63 years later, and it still sparkles.

Richard M. Gross is the corporate vice president of research and development and new business growth for Dow Chemical Co. He has a bachelor's degree in chemical engineering from Worcester Polytechnic Institute and a doctorate in chemical engineering from the University of Utah.

PLATINUM AT A GLANCE

Name: From the Spanish platina, silver.

Atomic mass: 195.08.

History: Known since ancient times. The pre-Columbian Indians of South America used platinum, but it wasn't noticed by Western scientists until 1735. Credit for its modern rediscovery is usually given to Antonio de Ulloa.

Occurrence: Occurs naturally in its native form in the Urals in Russia, as well as in Canada, South America, Colombia, and Peru. Platinum can also be extracted as a by-product of copper and nickel refining.

Appearance: Silvery white, solid metal. Lustrous, malleable, and ductile.

Behavior: Very resistant to corrosion and not volatile. Platinum salts are toxic.

Uses: Used in its pure form in jewelry since it is long-wearing and extremely white. Platinum is widely used in catalysts and has found great use in automobile catalytic converters. It is also used in semiconductors and is a component of some anticancer drugs.

DARMSTADTIUM AND BEYOND

DARLEANE C. HOFFMAN, UNIVERSITY OF CALIFORNIA, BERKELEY

More than 150 years elapsed between the discovery of uranium (92) in 1789 and neptunium (Np, 93), the first transuranium element, in 1940. Now, 110 elements have been identified, named, and placed in the periodic table. No longer are there four "missing" elements in the periodic table, as there were when the first "artificial" element, technetium (43), was produced and identified in 1937, just before World War II. Uranium was then the heaviest element known, and the elements astatine (85, discovered in 1940) and francium (87, 1939) were missing from the body of the table and promethium (61, 1945) was missing from the lanthanides. It is awe-inspiring to realize that those four missing elements, plus the 11 actinides from Np through lawrencium (Lr) and the first six transactinides from rutherfordium (Rf) through meitnerium (Mt), have been added to the periodic table since 1937 when Tc was produced! These 21 elements have increased the number of known elements by nearly 25% in 66 years.

Discoveries of three transactinides--elements 110, 111, and 112--were reported between 1995 and 1996. A Joint Working Party (JWP) of the International Union of Pure & Applied Chemistry (IUPAC)/International Union of Pure & Applied Physics was appointed to consider the discovery claims; the evidence has been deemed sufficient for elements 110 and 111, and the GSI discoverers have been invited to propose "real" names to replace the three-letter symbols and "systematic names" based on 0 = nil, 1 = un, 2 = bi, 3 = tri, and so mandated by IUPAC in 1979. Many of you will no doubt recall hearing Glenn T. Seaborg's sonorous and humorous drawn-out pronunciation of the provisional designations for elements 110 (Uun) and 111(Uuu) as "oon-oon' NIL-i-em" and "oon-oon' OON-i-em," much to the amusement of most of us working in this field who simply used the atomic numbers, thus avoiding these designations. Darmstadtium was approved by IUPAC as the official name for element 110 on Aug. 16, 2003.

The production and identification of Np and the rest of the actinides were accomplished rather quickly, as shown in the timeline. However, all the elements after mendelevium (Md, 101) were first positively identified using "physical" rather than chemical-separation techniques, ushering in a period of controversy. Methods other than the classical chemical-separation techniques had to be devised for positive determination of the atomic number (Z) of these short-lived elements, which were produced in quantities of only a few atoms by bombardment of heavy targets with high-intensity projectiles.

Positive identification of the first transactinides, Rf and dubnium (Db) [or Ha; many publications of chemical studies prior to 1997 use hahnium (Ha) for element 105], was delayed until 1969–70 as scientists worked to develop new techniques to produce and identify these elements. New methods based on - correlation to link the new element to known daughter nuclides were developed. Spontaneous fission rather than -decay is often the dominant decay mode and, although relatively easy to detect, it effectively destroys information about the atomic number and mass number of the original nucleus and has led to much controversy concerning identification of the atomic number of the fissioning nuclide.

Element 106 was soon produced at the Heavy Ion Linear Accelator (HILAC) at Berkeley (1974) by a joint Berkeley/Livermore group, but a long time elapsed before elements 107 through 109 were reported in 1981–84 by researchers using new production methods and the velocity separator SHIP at the Universal Linear Accelerator (UNILAC) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany. More than 10 years passed while the GSI groups made further improvements in SHIP and UNILAC before production of elements 110 through 112 was reported in 1995 and 1996.

Darmstadtium. Four different isotopes of element 110 were initially reported by three different groups. A single event of ²⁶⁷110 produced in the ²⁰⁹Bi(⁵⁹Co,n) reaction was reported in 1995 by a group led by Albert Ghiorso at the Lawrence Berkeley National Laboratory (LBNL). Evidence for isotopes ²⁶⁹110 and ²⁷¹110 produced in reactions of lead (Pb) and bismuth (Bi) targets with ^62,64Ni projectiles was reported in 1995 by a group led by Sigurd Hofmann at GSI. A single event of ²⁷³110 produced in the ²⁴⁴Pu (³⁴S,5n) reaction was reported in 1996 by a joint Dubna/Livermore group led by Yu. A. Lazarev at the Joint Institutes for Nuclear Research in Russia. Each of these groups used different production reactions, so none constituted confirmation of the others.

However, in 2002 the GSI group reported confirmation of their own results and the production of additional isotopes of 110, ^269,271110. Groups at LBNL and at the Japan Atomic Energy Research Institute have also confirmed the earlier GSI results. The LBNL group was unable to confirm their initial result because the SuperHILAC was shut down permanently after the experiment was finished, and the Dubna/LLNL group did not conduct additional experiments to confirm its report. After consideration of all the results, the JWP said that clearly the GSI group should be acknowledged as the discoverers. The GSI group was invited by IUPAC to propose a name for element 110 and chose darmstadtium (Ds) after Darmstadt, near the site where the research was conducted. The Inorganic Chemistry Division of IUPAC submitted the proposed name to the IUPAC Bureau and Council for final approval at the IUPAC General Assembly in Ottawa last month.

Elements 111 and 112. The GSI group initially reported discovery of elements 111 and 112 in 1995 and 1996, respectively, but JWP found that the data were insufficient to constitute discovery. Additional results designed to confirm the initial investigations were published by the GSI group in 2002, and JWP has now completed its examination of the additional data. The findings have been reviewed, and the summary should soon be published on the IUPAC Web page (http://www.iupac.org) for comment. JWP found that the additional evidence presented for element 111 is sufficient to constitute confirmation of the discovery by the GSI group, but deemed that the data for element 112 are still not sufficient.

Attempts by the GSI group to produce element 113 using similar methods were unsuccessful, and extrapolation from their previous experiments convinced them that production rates for the elements beyond 112 using Pb and Bi targets had dropped so low that further increases in the efficiency of their systems or perhaps use of different target-projectile combinations would be required. Production of element 113 has not yet been reported.

Now that production of the elements 110 and 111 has been confirmed and confirmation of 112 may soon follow, what about their chemistry? The half-lives of the longest confirmed isotopes are only 55 milliseconds (ms) for 110, 2 ms for 111, and 0.6 ms for the reported isotope of 112, hardly promising candidates for chemical studies. A similar situation exists for Mt, whose longest known isotope is only about 40 ms. Some longer lived isotopes of elements 110 and 112 have been reported by researchers at Dubna in 2000–02, but none of these has yet been confirmed.

An isotope with a half-life of at least a second is needed for chemical studies, and its decay characteristics must be well established in order to furnish a positive "signature" to prove that the desired element is actually being studied. Because the production rates are so low--often only decay of a few atoms per week can be detected--the results of many separate identical experiments must be combined. Very efficient chemical separations must be devised that reach equilibrium rapidly, can be conducted in a short time compared to the half-life, and give the same results on an "atom-at-a-time" basis as for macroquantities.

Typically, the isotope with the longest half-life and largest production rate is chosen for chemical studies; this is not necessarily the first isotope discovered. For example, the half-lives of the isotopes used in the first definitive chemical studies of the transactinides are 75 seconds (²⁶¹Rf), 34 seconds (²⁶²Db), 21 seconds (²⁶⁶Sg), 17 seconds (²⁶⁷Bh), and about 14 seconds (²⁶⁹Hs). [The predictions of deformed shells at Z = 108–110 and number of neutrons (N) = 162–164 spurred experimentalists to search for longer lived isotopes of seaborgium (Sg), bohrium (Bh), and hassium (Hs) for chemical studies; these were discovered and used in the first chemical studies.] Their well-known a-decay properties were used for positive identification.

The improvement in experimental techniques for atom-at-a-time studies of elements with both short half-lives and small production rates has permitted chemical studies of both aqueous- and gas-phase chemistry of the transactinides through Sg. In general, these studies have confirmed that their chemical properties are similar to those of their lighter homologs in groups 4, 5, and 6, respectively; however, unexpected deviations from simple extrapolation of known trends within the groups were found. Theoretical investigations based on molecular relativistic calculations helped provide guidance for experimentalists in designing these experiments.

Bh and Hs have only been studied in the gas phase. Studies of the oxychloride of Bh reported in 2000 showed that it behaved similarly to rhenium and technetium, and in 2002 separation of Hs as a volatile oxide similar to that of osmium tetroxide was reported. Solution chemistry of Bh and Hs has not been conducted because the aqueous chemistry and preparation of samples suitable for measuring a-particles or fission fragments is too slow. Very fast liquid-liquid extraction systems followed by direct incorporation of the activity in a flowing-liquid scintillation detection system have been developed for Rf and Db and may prove applicable in future studies of solution chemistry of short-lived species. Another promising technique is the use of a "preseparator" such as the Berkeley Gas-filled Separator (BGS) to separate and furnish the desired heavy-element isotope to the chemical system so that studies of its chemistry do not first require decontamination from other recoiling products.

These results for the lighter transactinides suggest that Mt, Ds, and elements 111 and 112 should be placed under iridium, platinum, gold, and mercury, respectively, as members of the 6d transition series that began with Rf and is expected to end with element 112. Mt and Ds have received little recent attention from chemical theorists, as 111 and 112 have been deemed more interesting. Relativistic calculations indicate that in element 111, the +3 and +5 states will be more stable than in gold and that the +1 exhibited by gold may be very difficult to prepare. Element 112 is the most interesting, as the calculations predict that the strong relativistic contraction and stabilization of the 7s orbitals and its closed-shell configuration should make element 112 rather inert and that it may actually behave more like a rare gas and have properties more similar to radon than to mercury. Experiments have been designed to try to determine whether 112 behaves more like mercury or radon, but definitive results must await discovery of a longer lived a-decaying isotope in order to establish that element 112 is actually being investigated.

Superheavy Elements. The transactinides are defined simply as all those beyond Lr, so, of course, this includes the long-sought "island" of superheavy elements (SHEs) predicted in the 1970s to be near the spherical nuclear shells at Z = 112–114 and N = 184. Half-lives as long as billions of years were calculated, and the island was believed to be separated by a "sea of instability" from the peninsula of known nuclei.

Production of element ²⁸⁸114 (174 neutrons) with a half-life of about three seconds and element ²⁹²116 (176 neutrons) with a half-life of ~50 µs was reported by a Dubna/LLNL collaboration working at Dubna in 2000–02 using ²⁴⁴Pu and ²⁴⁸Ca targets with ⁴⁸Ca projectiles, but the results have yet to be confirmed. They have proposed that these should be called SHEs, although they are still far from the 184-neutron shell. The element 112 and 110 daughters of these -decay chains were reported to have half-lives on the order of 10 seconds. It is extremely important to confirm these results, because investigations of their chemical properties could then be considered if the production rates can be increased.

Some nuclear theorists predict that element 110 with N = 182–184 might be the longest lived SHE, with a half-life of about 100 years. Others propose that the strongest spherical shell might be at Z = 124 or 126, while still others suggest Z = 120 and N = 172. At any rate, it now seems clear that species with half-lives long enough for chemical studies can exist all along the way to an "island of stability"; the critical problem is how to synthesize them.

Reactions of Pb or Bi with ⁸⁷Rb or ⁸⁶Kr projectiles could produce ²⁹⁴119, whose half-life would only be microseconds, but it would decay to new longer lived odd-Z elements 117, 115, and 113 via successive a-emission and end with a known Lr isotope. Some theorists have suggested that so-called unshielded reactions in which the Coulomb barrier is below the bombarding energy might result in enhanced production yields of very neutron-rich nuclei. For example, the reaction ¹⁷⁰Er(¹³⁶Xe, n) would make ³⁰⁵122 with 183 neutrons. Reactions using radioactive beams such as ⁴⁷K or ⁴⁶Ar with radioactive actinide targets have been suggested, but even these still don't reach the Z = 114, N = 184 region. Another possibility is to use accelerated, neutron-rich fission product beams of krypton, rubidium, and strontium with Pb and Bi targets to reach the Z = 120 and 126 region. Not only must new reactions be investigated, but imaginative methods to increase beam intensities and production rates and even to develop new kinds of accelerators and projectiles must be found if we are to produce these longer lived isotopes that lie just beyond our reach.

New automated systems with preseparators may be envisioned to facilitate both aqueous- and gas-phase studies of short-lived isotopes, and techniques to "stockpile" possible longer lived species at the accelerators where they are produced for later off-line separation must be devised if this tantalizing new region of elements is to be explored.

Darleane C. Hoffman is a professor of the graduate school of chemistry at the University of California, Berkeley, and faculty senior scientist in the Heavy Element Group of the Nuclear Science Division of Lawrence Berkeley National Laboratory. She was awarded the National Medal of Science in 1997, the ACS Priestley Medal in 2000, and the 2003 Sigma Xi William Procter Prize.

DARMSTADTIUM AT A GLANCE

Name: Named after Darmstadt, Germany, site of Gesellschaft für Schwerionenforschung (GSI).

Atomic mass: 269

History: Three different groups reported producing element 110 in 1995-96, but discovery was credited to the GSI group, which was the first to confirm their results. The element was formally named only last month.

Occurrence: Artificially produced.

Appearance: Solid of unknown color.

Behavior: Highly radioactive.

Uses: No commercial uses.

COPPER

GEORGE M. WHITESIDES, HARVARD UNIVERSITY

I fell in love with copper as an assistant professor. How did it happen? By happenstance flavored with a soupçon of sloth, and after an offhanded introduction.

While a graduate student at California Institute of Technology, I worked with organolithium reagents (RLi). As the time to graduate approached, I needed my own problem to work on. At that time, as little was known about the properties of s-bonded transition-metal alkyls as about the mating habits of kraken, and for the same reason--few of either had been reliably observed. There was a radical idea that such organometallic compounds might somehow be involved in catalysis, and their descriptive chemistry was essentially unexplored. The area was appealing, but the question was, "Where to start?"

Several organometallic compounds of gold were known, and to my unsophisticated eye, "RAu" looked vaguely and comfortingly like "RLi." I skimmed the literature of organogold chemistry briefly (very briefly, since there was almost none--a great advantage, I thought, since libraries were not my thing) and decided that gold would be a great place to begin exploring transition-metal alkyls. Then, one day, my research director, Jack Roberts, ambled by and asked what I was going to start on at Massachusetts Institute of Technology. I replied, "Gold organometallic chemistry." He considered this idea for 30 seconds, then mumbled "No. Don't do that. Work on copper--it's more interesting--and cheaper," and wandered off. And that was that--the die was cast.

STUNNING Crystals of copper sulfate (grown by Elizabeth Tran), composed on copper foil, with a section of copper wire. PHOTO BY FELICE FRANKEL AND ELIZABETH TRAN

He was right, of course. As I discovered over the following years, copper was a kind of pixie dust for organic chemistry. It turned fireworks and flames green; it made Grignard reagents do conjugate additions; it was essential for the Sandmeyer and Ullman reactions; it coupled acetylenes; it was a component of exotic heterogeneous catalysts; it promoted apparent nucleophilic substitution reactions on aromatic halides; it catalyzed autoxidation reactions. None of these reactions were understood mechanistically, but all were probably different. That kind of mechanistic chaos was, of course, candy to an aspiring mechanistic organometallic chemist.

At MIT, after considerable technical difficulty (largely due to my experimental incompetence), I finally found a copper(I) salt [(CH₃)₂SCuI] that was soluble in cold ether and added CH₃MgI to it. A stunningly beautiful (to me), bright yellow, fluffy precipitate immediately formed, which, when warmed, turned black. What else could it be but CH₃Cu? And copper metal on decomposition? Such pleasure!

Organic reactions involving copper are interesting, but they are a tiny part of the long and complicated relationship between this element and our species. We are, of course, ourselves part copper: Copper-containing enzymes are essential catalysts for a number of redox reactions. We use copper--especially copper metal--in many ways. Copper salts are easily reduced to copper metal, and copper is one of the few metals that occur in large quantities in nature. (The occasional availability of large lumps of native copper contributed to the charming sociopathology of potlatch, highly refined among the Kwakiutl of the Pacific Northwest.) Pure copper is suitable for working into ornamental objects but too soft to use for serious mischief. That deficiency was fixed by alloying copper with tin to give bronze, an excellent alloy for spear tips and swords, and an early example of the willingness of our species to exercise high levels of technological creativity in the service of weaponry. (Of course, bronze was also useful for kettles and spoons, but a spear in the room fixes the attention.)

Silver is valuable but too soft to make durable coins: Alloying it with a little copper gives sterling silver. Copper sheeting makes excellent roofs, splendid in their red-gold color when new, and soothing in the soft, mottled green of verdigris when they age. And copper wire! The world (at least the electrical world) is, in a sense, built of it. Copper is an excellent and ductile electrical conductor, easily drawn into the wire used in countless circuits--from the wiring of houses to the armatures of motors. Electroplating and vapor deposition have now also made it a part of microprocessors. It has appeared yet again in high-T_c superconductors. Who could measure the aggregated magnetic fields generated by electrons streaming through copper wire?

In the plays acted by the elements, copper is a bit player, but one with indispensable roles--as a component of materials, reagents, and catalysts; as a metal; and as a part of life.nd I still love it, but now I've gone back to gold.

George M. Whitesides is Mallinckrodt Professor of Chemistry at Harvard University. He was a member of the MIT chemistry faculty from 1963 to 1982 and joined Harvard in 1982. At the beginning of his career, he worked on organocopper chemistry; now, among other things, he workson self-assembled monolayers (SAMs) on gold.

COPPER AT A GLANCE

Name: From the Latin cuprum, "from Cyprus." The island was a source of the metal for the Romans.

Atomic mass: 63.55.

History: Known to many ancient civilizations.

Occurrence: Somewhat rare, making up only 0.0007% of Earth's crust.

Appearance: Reddish, soft metal.

Behavior: Copper is malleable, ductile, and a good conductor of heat and electricity.

Uses: Essential in trace amounts to living systems. Copper is used in water pipes, bronze statues, and bells; commonly used for electrical wires. It helps form the alloys brass and bronze.

SILVER

ALAN SHAW, CODEXIS

I was first drawn to the magic of silver's luster while standing as a child in the British Museum, gazing at a finely polished metallic silver mirror and asking, "Why?" Not having the benefit, at the time, of a college education and a second degree majoring in transition-metal chemistry, the question went unanswered.

Today, however, silver's time-honored and unique ability to reflect almost 100% of the light that falls on it is, for me, more easily explained. It's a result of teamwork between light and the outermost single electron. This electron, when activated by light, absorbs energy that enables it to jump into a higher, faster orbit around the atomic nucleus. This new orbit cannot, however, be permanently sustained--the attraction of the nucleus is too great. As the electron falls back to its original orbit, its absorbed energy is radiated outward in the form of light, resulting in the eye-pleasing radiance known as luster.

To the ancients, however, silver was an object of wondrous beauty, imbued with the mystic qualities of the moon, whose dark and mysterious nature it seemed to mirror. My personal fascination with silver began through admiration of Greek and Roman artifacts, but it certainly did not end there. My love of all things ancient revealed, after years of study, a fundamental, almost deific, relationship between man and this indispensable metal that has perpetuated into modern times, albeit in a more subtle form.

IN HIS IMAGE Alexander the Great minted silver coins emblazoned with his image.

The Druids of the ancient British Isles harvested holy mistletoe and oak leaves with finely engraved and enchanted silver sickles. The very shape of these swords is reflective of the image of the crescent moon, the heavenly body silver was believed to represent on Earth. Alexander the Great, a devotee of the Olympian Pantheon, believing himself to be a god in life, was after death worshiped in association with the sun god Helios. Was there a connection between this cult and the gifting of highly polished and quite valuable silver shields to Alexander's most seasoned veterans, the Hypaspists, an elite group within the Agema (royal guards)? This group came to be known as the Agryaspids, or Silver Shields, and became as famous as Caesar's 10th Legion or Napoleon's Old Guard are to us now.

One can visualize the scene as an enemy column advanced toward the Macedonian line, when, on Alexander's command, the shields and the full power of the sun's rays were brought to bear. It is not hard for me to imagine Alexander claiming the authority and divinity of the sun at such a time--hence, perhaps, his association with it after life.

The relationship between silver and the history of humankind has endured. Silver has a number of unique properties that set it apart from other metals. These include strength, malleability, and ductility; electrical and thermal conductivity; and, as already referenced, high reflectance of light and a lesser known but increasingly important medical application as a bactericide.

History reflects man's almost lustful quest to acquire silver, and our modern vocabulary is replete with references to it. Indeed, over the years, silver, like no other metal, has become synonymous with beauty, wealth, style, health, and mystique. For example, the phrase "born with a silver spoon in your mouth" is a reference to wealth and health. In the 18th century, babies who were fed with silver spoons were found to be healthier than those fed with spoons made from other metals, and silver pacifiers have subsequently found wide use in the U.S. because of their beneficial health effects.

To ask for silver service would be to expect the very best. To find oneself looking for a silver lining is to hope for a benevolent outcome to an otherwise uncomfortable and adverse situation. A star of the silver screen is in part a reference to the use of silver in the film industry but also, again, a reflection of its association with beauty and style. Even in nature, the silver birch is regarded as a tree of elegance, commonly referred to in northern Europe as the "Queen of the Forest."

Finally, if you were a lycanthrope or indeed a vampire, according to myth, to be struck by a silver-tipped arrow or bullet would result in instant and irrevocable death. For me, the great author J. R. R. Tolkien so eloquently captured all of the aforementioned characteristics when describing the Mirror of Galadriel in the epic trilogy "Lord of the Rings."

We end as we began, with a reflection of man and his endless interaction with this most noble of metals. For me, silver represents true majesty among the elements. If gold claims to be its king, silver must surely be its queen.

Alan Shaw is president and chief executive officer of Codexis. He has had a long career in the fine chemicals industry, including with Clariant, Archimica (BTP plc), Chirotech Technology Ltd., and ICI.

SILVER AT A GLANCE

Name: From the Anglo-Saxon seolfor, silver. The symbol is from the Latin word for silver, argentum.

Atomic mass: 107.87.

History: Known since ancient times. Ancient slag dumps indicate that silver was separated from lead as early as 3000 B.C.

Occurrence: Silver occurs in ores including argentite and horn silver and in conjunction with deposits of ores containing lead, copper, and gold.

Appearance: White metallic solid.

Behavior: Silver has the highest electrical and thermal conductivity of all metals and possesses the lowest contact resistance. It is very ductile and malleable.

Uses: Used to make photographic film, tooth filings, silverware, mirrors, batteries, photosensitive glass, and as an electrical conductor. Silver iodide is used for seeding clouds to produce rain.

GOLD

ALAN LIGHTMAN, MASSACHUSETTS INSTITUTE OF TECHNOLOGY

At first, he stood behind the green beaded curtain, simply watching her. In the far corner of the room, she knelt on the floor and worked slowly on a small woven basket. Every few moments, she paused to straighten her back. Her bare arms glowed in the flickering light of the sesame oil lamp. To him, she seemed sad. What was he doing here? he thought to himself. Better to have stayed at his farm, where he could get soused with his sheep, count stars in the hard empty night. He started to turn, but his foot caught a hanging mat.

"Keb!" Her basket dropped to the floor.

Now that he saw her full face, she was even more beautiful than he remembered. Her high cheekbones, the long curve of her neck, her lips. She wore a tapered robe of white linen, held at her shoulders by delicate thin straps, and he could see the outline of her breasts beneath the cloth. Awkwardly, he wiped his mouth with his sleeve. His tunic smelled of goat, onions, and manure.

"Keb," she repeated. Her hands trembled, like his.

He fumbled with something in his pocket. Could he give it to her now? He wanted to flee, but he also wanted to ride out this sudden bursting of flutes in his brain, he wanted for once in his life to do something right, he wanted to make love to her. His eyes moved hesitantly around the room, to a bowl of pomegranates on a table, a terra-cotta jar, a single narrow window with its painted wood grille. She was born in this house. Nineteen years she had lived in this house. Her younger sisters were already married.

He would give it to her now, the necklace, made of the miraculous yellow metal that was both warm and cool to the touch, that never tarnished, that could be hammered into a sheet as thin as a layer of sunlight on water. The necklace had cost him 27 sacks of radishes and spinach. He would have paid a hundred. No one had ever seen such a necklace.

"It's for you." He held out the gift. It glinted and gleamed in the light.

She took one step toward him. It seemed to him that she looked at him with pity.

"Please," he said. How he wanted to fasten that necklace around the curve of her neck, to hold her. He imagined taking her away from this small stifling house, teaching her to paint, brushing her hair in the evening. He had inherited property, he had become something now. A slight breeze came through the window and rustled the beads. Somewhere, he heard chimes. Voices whispered in another part of the house. Then the room was silent again, so silent that he could hear the minute scratchings of a cicada crawling across the stone floor and a muffled thumping that must have been the beating of his heart. Would she not come closer? Why did she look at him that way? She was a silhouette now, standing in front of the single oil lamp. He could smell the lotion of terebinth on her skin.

"It is beautiful," she said, looking at him. "Give it to someone else." She touched his hand for a moment, then returned to her basket.

Outside, he threw the necklace into the darkness. Never. He slammed his hand into a stone post and watched as the dark blood trickled to the ground.

Alan Lightman, author of "Einstein's Dreams," is a physicist and novelist and adjunct professor of humanities at Massachusetts Institute of Technology. His most recent book is the novel "Reunion."

GOLD AT A GLANCE

Name: From the Anglo-Saxon geolo, yellow. The symbol comes from Latin aurum, shining dawn.

Atomic mass: 196.97.

History: Known to ancient civilizations.

Occurrence: Found free in nature and associated with quartz, pyrite, and other minerals. Two-thirds of the world's supply comes from South Africa, and two-thirds of the U.S. supply is from South Dakota and Nevada. Gold is found in seawater, but no effective economic process has been designed yet to extract it from this source.

Appearance: Soft, shiny yellow metal that can also appear black, ruby, or purple when finely divided.

Behavior: Gold is the most malleable and ductile metal. It is unaffected by air and most reagents, and is a good conductor of heat and electricity. Gold is generally nontoxic.

Uses: Commonly used in jewelry, although it often needs to be alloyed with other metals to give it more strength. Gold alloys are also used to make decorative items, dental fillings, and coins. Because gold is a good reflector of infrared radiation, it can be used to help shield spacecraft and skyscrapers from the sun's heat. Aradioactive isotope of gold, gold-198, is used for treating cancer, and gold sodium thiosulfate is used to treat arthritis.

ZINC

RONALD BRESLOW, COLUMBIA UNIVERSITY

Zinc is an important element, especially because it plays a key role in many enzymes that are essential to life. For this reason, we humans consume approximately 20 mg of Zn(II) in our daily diets.

I have also had a series of more special interactions with zinc. Some years ago, my wife, Esther, was writing her master's thesis on the enzyme carbonic anhydrase and proposed the correct mechanism of the enzyme--its zinc acts both as a Lewis acid, by coordinating to a developing oxyanion, and as the coordination site for a basic hydroxide ion, the nucleophile that adds to the carbon dioxide molecule. Later, my research group synthesized zinc-based mimics of carbonic anhydrase and of carboxypeptidase. We also showed that carboxypeptidase itself uses the carbonic anhydrase type of mechanism.

Another example of the special properties of zinc in enzyme catalyst systems was waiting for me, but I didn't know it for a while. In 1974, Paul Marks, then dean of Columbia University's medical school, came to me with a remarkable story and challenge. Charlotte Friend of Mount Sinai School of Medicine had discovered that certain cancer cells--pre-erythrocytes that were infected with a virus (MEL cells)--were induced to differentiate to normal red blood cells in the presence of high concentrations of dimethylsulfoxide. We set out to see if we could produce much more potent compounds for this remarkable "reform" of cancer cells, learn how they worked, and perhaps even find whether they were useful with other types of cancer.

FITTING IN An X-ray structure of SAHA bound to the zinc of an HDAC.