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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, HARRY B. GRAY, CALIFORNIA INSTITUTE OF TECHNOLOGY colleagues and I used aqueous Cr(II) to transfer electrons to the blue copper cenWAS D E S T I N E D TO WORK ON C H R O the triply bonded oxoclirornium(V) unit, and ters in spinach plastocyanin, bean plasmium: My mother's name was Ruby Curtis R. Hare and I interpreted its d-d specMom loved the stunning red gemtrum [Inorg. Chem., 1,363 (1962)]. (Later, with tocyanin, Rhus vernicifera laccase, and stellacyanin [Proc. Natl. Acad. Sci. USA, 6 9 , stone, b u t she didn't know there Carl J. Ballhausen and V M. Miskowski, I 30 (1972)}. Our experiments would be a ruby laser, and neither did worked on the spectra of chroshowed that electron transfers I, during the late 1940s and early '50s mate and halochromates, which, to some copper centers were when I was experimenting with chromilike dichromate, are all in the VI much slower than others, and we um compounds in the basement of our oxidation state made famous by now know that in most biohouse. I had quite a nice lab, and I re- Julia Roberts in the movie "Erin logical reactions electrons must member that Mom was worried that I Brockovich.") tunnel through many bonds to would blow myself up, especially when I During the next year at Coreach their destinations. Much used dichromate as an oxidant in speclumbia University Nancy Beach later, in work reminiscent of tacularly exothermic redox reactions. and I worked on zerovalent CELEBRATING Taube's, Israel Pecht and Ole chromium: "We reported that the C&EN'S Farver identified one of t h e 80TH UV spectrum of its carbon monANNIVERSARY Cr(III)-protein binding sites afoxide complex, chromium hexater Cr(II) reduction. In so doing, carbonyl, exhibits two very inthey estimated how far an electron had to tense absorptions that are attributable to travel in its journey to a copper active site. metal-to-ligand charge-transfer (MLCT) Many investigators have explored the transitions. Amazingly our interpretation of spectroscopy and chemistry of the d-d exthe hexacarbonylMLCTspectrum has withcited states of octahedral Cr(III), and the stood 40 years of theoretical scrutiny alexcited doublet, in particular, has had a though the positions ofthe lowest d-d excitalong and glorious (some say checkered) histions have beenrevisedby density functional S E E I N G R E D Chromium impurities tory Whether the doublet undergoes astheorists. The excited-state dynamics of the in corundum give rubies their sociative substitution is still debated when molecule are nowunderstood in great detail, distinctive red color. "inorganikers" gather for discussions of and for that reason, zerovalent chromium mechanisms. In 1985, with Bruce S. Brunusually makes an appearance in textbooks In college, I worked on the analytical schwig and other colleagues, we examined chemistry of chromium and three of its the rates of oxidation of reduced blue-copneighbors: vanadium, niobium, and moCHROMIUM AT A GLANCE per proteins by this relatively long-lived lybdenum. Although I loved the chaland powerfully oxidizing Cr(III) reagent, lenge of analyses, the inorganic Name: From the Greek chroma, color. in one of the early demonstrations of elecresearch that Fred Basolo and Atomic mass: 52.00. tron tunneling through folded polypepRalph Pearson were doing at History: Discovered in 1780 by tides {Inorg. Chem., 24,3743 (1985)}. Northwestern University exNicholas Louis Vauquelin. cited me even more, and I All in all, I have worked on six oxidaOccurrence: Isolated mostly from joined their group in 1958. The tion states of chromium. In recent times, chromite ore. late '50s were the glory days of inand in collaboration with Zeev Gross of Appearance: Blue white, hard and britorganic mechanisms, and chromium was Technion-Israel Institute of Technology, tle metal. one of the big stars. The chromous ion, Behavior: Resists oxidation in air. Many our group has managed to prepare which is a powerful reductant and can atchromium compounds are toxic. Chrochromium corroles in four oxidatack oxidants at close range, is substitution mates are corrosive to skin and tistion states, I I I through VI, allabile; and, as all inorganic chemists know, sue. Chromium itself is a human poithough "VI" turned out to be V the chromium product of the reaction, son if ingested and is suspected of complexed to an oxidized corrole. Cr(III), is substitution inert. H e n r y being a carcinogen. The presence of The joint lechnion-California InTaube's famous experiment demonstratchromium impurities in gems is often restitute of Technology research on aering atom transfer from chloropentaamsponsible for their brilliant colors. obic oxidations catalyzed by chromium minecobalt(III) to chromous ion relied Uses: Essential trace element in hucorroles is the latest chapter in my affair on these properties of two of the more mans that plays a role in glucose mewith element 24. common oxidation states of the element. tabolism. In alloys, chromium can serve as a protective coating from oxidation In my work in Copenhagen in t h e Harry B. Gray is the Arnold O. Beckman Proand comprises up to 18% of stainless spring of 1961,1 became fascinatfessor of Chemistry at Caltech. He received the steel. Its oxides are often used as piged with oxo complexes, especially NationalMedal of Science in 1986 andthe Priestments, for tanning leather, and in highthe vanadyl and chromyl ions. The ley Medalin 1991; in 2003, he receivedthe Nichols quality recording tapes. and WhelandMedals as wellas the NationalAcadgarnet-red ammonium pentaemy ofSciencesAward in Chemical Sciences. chlorooxochromate(V) contains
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