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

was the result of combined works of Dubna and Berkeley; that element 104 (rutherfordium) was discovered contemporaneously and independently at Dubna (identification by chemistry) and Berkeley (by physical methods); that element 105 (dubnium) was discovered by physical techniques also contemporaneously and independently in the two laboratories; that element 106 (seaborgium) was discovered at Berkeley; and that the major credit for 107 to 109 (bohrium, hassium, and meitnerium) went to GSI Darmstadt, where research started in the late 1970s.

We were thrilled when IUPAC issued its recommendation to name element 105 dubnium "to recognize the distinguished contributions to chemistry and modern nuclear physics of the international scientific center."

In recent years, the Dubna research team, now headed by Yuri Oganessian, has reported major breakthroughs regarding the long-awaited "island of stability" around atomic number 114, where some of the heavy nuclei might live much longer than isotopes of elements 102 to 108. When bombarding uranium and transuranium (up to californium) targets with extremely intense beams of ⁴⁸Ca, the researchers discovered several new, relatively long-lived -and spontaneous-fission nuclides that can be assigned to elements 110 to 118! But this subject undoubtedly deserves a separate paper.

Ivo J. Zvara has been with the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Russia, since 1960. He was the leader of radiochemical studies of synthetic elements and pioneered gas-phase chemistry of transactinide elements.

DUBNIUM AT A GLANCE

Name: Named after Dubna, Russia, the site of the Joint Institute for Nuclear Research (JINR).

Atomic mass: (262).

History: A team from JINR first reported producing dubnium in 1967. In 1970, both the Russian team and a team from Lawrence Berkeley National Laboratory confirmed the discovery.

Occurrence: Does not occur naturally.

Behavior: Highly radioactive.

CHROMIUM

HARRY B. GRAY, CALIFORNIA INSTITUTE OF TECHNOLOGY

I was destined to work on chromium: My mother's name was Ruby. Mom loved the stunning red gemstone, but she didn't know there would be a ruby laser, and neither did I, during the late 1940s and early '50s when I was experimenting with chromium compounds in the basement of our house. I had quite a nice lab, and I remember that Mom was worried that I would blow myself up, especially when I used dichromate as an oxidant in spectacularly exothermic redox reactions.

In college, I worked on the analytical chemistry of chromium and three of its neighbors: vanadium, niobium, and molybdenum. Although I loved the challenge of analyses, the inorganic research that Fred Basolo and Ralph Pearson were doing at Northwestern University excited me even more, and I joined their group in 1958. The late '50s were the glory days of inorganic mechanisms, and chromium was one of the big stars. The chromous ion, which is a powerful reductant and can attack oxidants at close range, is substitution labile; and, as all inorganic chemists know, the chromium product of the reaction, Cr(III), is substitution inert. Henry Taube's famous experiment demonstrating atom transfer from chloropentaamminecobalt(III) to chromous ion relied on these properties of two of the more common oxidation states of the element.

In my work in Copenhagen in the spring of 1961, I became fascinated with oxo complexes, especially the vanadyl and chromyl ions. The garnet-red ammonium pentachlorooxochromate(V) contains the triply bonded oxochromium(V) unit, and Curtis R. Hare and I interpreted its d-d spectrum [Inorg. Chem., 1, 363 (1962)]. (Later, with Carl J. Ballhausen and V. M. Miskowski, I worked on the spectra of chromate and halochromates, which, like dichromate, are all in the VI oxidation state made famous by Julia Roberts in the movie "Erin Brockovich.")

During the next year at Columbia University, Nancy Beach and I worked on zerovalent chromium: We reported that the UV spectrum of its carbon monoxide complex, chromium hexacarbonyl, exhibits two very intense absorptions that are attributable to metal-to-ligand charge-transfer (MLCT) transitions. Amazingly, our interpretation of the hexacarbonyl MLCT spectrum has withstood 40 years of theoretical scrutiny, although the positions of the lowest d-d excitations have been revised by density functional theorists. The excited-state dynamics of the molecule are now understood in great detail, and for that reason, zerovalent chromium usually makes an appearance in textbooks that have sections on the photochemistry of metal complexes.

The chromous ion is very special to me, as it launched my work on electron tunneling through proteins. In the early '70s, colleagues and I used aqueous Cr(II) to transfer electrons to the blue copper centers in spinach plastocyanin, bean plastocyanin, Rhus vernicifera laccase, and stellacyanin [Proc. Natl. Acad. Sci. USA, 69, 30 (1972)]. Our experiments showed that electron transfers to some copper centers were much slower than others, and we now know that in most biological reactions electrons must tunnel through many bonds to reach their destinations. Much later, in work reminiscent of Taube's, Israel Pecht and Ole Farver identified one of the Cr(III)-protein binding sites after Cr(II) reduction. In so doing, they estimated how far an electron had to travel in its journey to a copper active site.

SEEING RED Chromium impurities in corundum give rubies their distinctive red color.

Many investigators have explored the spectroscopy and chemistry of the d-d excited states of octahedral Cr(III), and the excited doublet, in particular, has had a long and glorious (some say checkered) history. Whether the doublet undergoes associative substitution is still debated when "inorganikers" gather for discussions of mechanisms. In 1985, with Bruce S. Brunschwig and other colleagues, we examined the rates of oxidation of reduced blue-copper proteins by this relatively long-lived and powerfully oxidizing Cr(III) reagent, in one of the early demonstrations of electron tunneling through folded polypeptides [Inorg. Chem., 24, 3743 (1985)].

All in all, I have worked on six oxidation states of chromium. In recent times, and in collaboration with Zeev Gross of Technion-Israel Institute of Technology, our group has managed to prepare chromium corroles in four oxidation states, III through VI, although "VI" turned out to be V complexed to an oxidized corrole. The joint Technion-California Institute of Technology research on aerobic oxidations catalyzed by chromium corroles is the latest chapter in my affair with element 24.

Harry B. Gray is the Arnold O. Beckman Professor of Chemistry at Caltech. He received the National Medal of Science in 1986 and the Priestley Medal in 1991; in 2003, he received the Nichols and Wheland Medals as well as the National Academy of Sciences Award in Chemical Sciences.

CHROMIUM AT A GLANCE

Name: From the Greek chroma, color.

Atomic mass: 52.00.

History: Discovered in 1780 by Nicholas Louis Vauquelin.

Occurrence: Isolated mostly from chromite ore.

Appearance: Blue white, hard and brittle metal.

Behavior: Resists oxidation in air. Many chromium compounds are toxic. Chromates are corrosive to skin and tissue. Chromium itself is a human poison if ingested and is suspected of being a carcinogen. The presence of chromium impurities in gems is often responsible for their brilliant colors.

Uses: Essential trace element in humans that plays a role in glucose metabolism. In alloys, chromium can serve as a protective coating from oxidation and comprises up to 18% of stainless steel. Its oxides are often used as pigments, for tanning leather, and in high-quality recording tapes.

MOLYBDENUM

PHILIP C. H. MITCHELL, UNIVERSITY OF READING, ENGLAND

I was introduced to molybdenum in the 1950s by R. J. P. Williams, my Oxford D. Phil. supervisor. We knew that the enzymes nitrogenase (which catalyzes the reduction of nitrogen to the ammonium ion), nitrate reductase (nitrate to nitrite), and xanthine oxidase (hydroxylation of xanthine to uric acid) were molybdenum dependent. At the Chester Beatty Cancer Research Institute in London, R. C. Bray was using electron spin resonance (ESR) to investigate the kinetics of xanthine oxidase. ESR showed that molybdenum was coordinated by sulfur.

Subsequently, X-ray crystallographers showed that the oxidase enzymes are built around oxomolybdenum centers ligated with sulfur. The structures are familiar from the model oxomolybdenum sulfur complexes that I and others studied. My group prepared the first Mo-cysteine complex, Na₂[Mo^V₂O₄{SCH₂CH(NH₂)COO}₂]5H₂O. However, the dimeric anion turned out to be a poor model of monomeric molybdenum in enzymes. What could not have been predicted were the unique Mo-Fe-S clusters central to nitrogenase. Researching the Mo-enzyme chemistry greatly extended knowledge of molybdenum coordination chemistry.

My first task as a graduate student was to get a feel for molybdenum chemistry. Back then, the source was Nevil V. Sidgwick's classic "The Chemical Elements and Their Compounds." The analytical chemistry of molybdenum--gravimetrically as lead molybdate, PbMoO₄, or the 8-hydroxyquinoline complex, [Mo^VIO₂(C₉H₆NO)₂]; colorimetrically as the purple, diamagnetic thiocyanate, [Mo^V₂O₃(NCS)₆]^2–, or the emerald green toluene-3,4-dithiolate, [Mo(C₇H₆S₂)₃]; volumetrically via reduction to Mo(V) or Mo(III)--also provided insights. Molybdenum is extraordinarily versatile: It forms compounds with most inorganic and organic ligands and has oxidation states from (–II) to (VI) and coordination numbers from 4 to 8. Therein lies its challenge and excitement.

The chemistry of molybdenum in its higher oxidation states (IV to VI) is dominated by oxo-species--molybdates [Mo^VIO₄]^2–; poly- and heteropolymolybdates; and, in complexes, Mo^VIO₂, Mo^VO, Mo^V₂O₃, Mo^V₂O₄, Mo^IVO, and Mo^IVO₂ as central cations. The oxide MoO₃, the molybdenum blues, the polymolybdates, and the remarkable molybdenum wheels of Achim Muller are built from linked [MoO_x] polyhedra. Oxomolybdenum redox chemistry is exploited in selective oxidation catalysis: In the oxidase enzymes and the heterogeneous catalysts bismuth molybdate and iron molybdate, molybdenum shuttles between oxidation states (VI) and (IV) while transferring O or HO to substrate molecules [J. Inorg. Biochem., 28, 107 (1986)].

Molybdenum has an extensive sulfur chemistry. The sulfide MoS₂ is the main molybdenum ore. Its layer structure confers lubricating properties like graphite to it. Complexes of dithiocarbamates, R₂NCS₂^–, and of dithiophosphates, (RO)₂PS₂^–, are used as oil-soluble lubricant additives, decomposing at rubbing surfaces to MoS₂. My group developed Mo-S chemistry in contracts with Shell and, later, with Esso, synthesizing many Mo-S compounds and lubricant additives [Wear, 100, 281 (1984)]. The Mo-dithiolate complex was an excellent friction modifier and wear reducer (and the colored oil a beautiful green) but was expensive. The water-soluble Mo-cysteine complex has potential in metalworking applications.

VIEW FROM THE EDGE Computer-generated picture of MoS2.