NEWS OF THE WEEK SCIENCE
PALLADIUM PUZZLE RESOLVED Suspected Pd(VI) turns out to be Pd(II), but compound is still 'remarkable' ALTERED STATE According to new theoretical calculations, unusual complex contains Pd coordinated to sigma Si-Si bonds.
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ists published a startling report: They claimed to have synthesized a complex that contained palladium(VI), a high oxidation state not seen before nor expected in this metal. The complex contained a central Pd, presumed to be Pd(Vl), surrounded by six silicon atoms. This was highly unexpected because silicon isn't thought of as an electronegative ligand [Science, 295, 308 (2002); C&EN, Jan. 14, page 8}. However, Yale University chemistry professor Robert H.
Crabtree, who wrote a perspective accompanying the report, and the Japanese group, which included Shigeru Shimada at the National Institute of Advanced Industrial Science & Technology in Tsukuba, entertained an alternate scenario. Two pairs of the silicon atoms were close enough together to possibly form sigma bonds with which the central Pd might be interacting. Although this would assign the Pd an oxidation state of II, not VI, the structure would still be unprecedented. Now two independent theoretical papers make a strong case for the Si-Si sigma bond/Pd(II)
AGRICULTURE
ORGANIC FARMING Comparison with conventional methods finds lower energy use, healthier soils
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from organic farming are lower than those achieved - with conventional methods, a long-term DOWN ON THE FARM At comparison shows. experimental plots in Therwil, But the organic sysSwitzerland, organic and tems are more efficonventional farming systems have been compared since 1978 cient overall in terms of nutrient and energy input. And they result in more fertile soils supporting greater biodiversity, Swiss researchers conclude. The 21-year study compared conventional plots, which used mineral fertilizers and synthetic pes8
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ticides, to organic plots fertilized with manure and treated only occasionally with a copper fungicide. Identical varieties of potatoes, winter wheat, grass clover, barley, and beets were grown using the same crop rotation and tillage practices [Science, 2 9 6 , 1694(2002)}. 'The organic systems are more efficient," says Andreas Fliessbach, a soil ecologist with the Research Institute of Organic Agriculture in Frick, Switzerland. "The yields were only 2 0 % less, even with nutrient input reduced by 50%." Also, the organic systems used 20 to 56% less energy when the amount required for production of fertilizers and pes-
scenario. Groups headed by chemistry professors Santiago Alvarez at the University of Barcelona and Christopher J. Cramer at the University of Minnesota, Minneapolis, say both electronic structure and geometric calculations show that Pd isn't actually oxidized by Si but interacts with S i - Si sigma bonds after reaching an oxidation state of II [Angew. Chem. Int. Ed., 41,1953 and 1956 (2002)}. "Such sigma-donor bonding of two Si-Si bonds is remarkable because these bonds have previously been seen only for Si-H, C - H , and other X - H systems," Crabtree notes. The new work should pave the way for the synthesis of classes of similar compounds that could be useful for numerous applications, including catalysis. "We hope further contributions by theoretical chemists will help us understand the mechanism of formation of these Pd-Si complexes," Shimada adds.—ELIZABETH WILSON
ticides was taken into account, he says. The researchers observed diverse communities of animals, insects, weeds, and microorganisms thriving in the organic plots. And the structure of the soil in the conventional plots is inferior, Fliessbach says, forming crusts after a rain, for example. CropLife America, the trade organization that represents pesticide manufacturers, sees flaws in the study, however. "There is no mention nor measure of the human labor involved in the respective farming systems, the comparative quality of crops harvested, nor the potential benefits for organic plots of pest suppression in adjacent conventional plots," says Ray McAllister, vice president for science and regulatory affairs. "This research was commenced, conducted, and concluded with a predetermined outcome: that organic farming is better."-PAMELAZURER
HTTP://PUBS.ACS.ORG/CEN
