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SCIENCE
The complex contained a central Pd, presumed to be Pd(VI), 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) 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 Si-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. |
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