CHEMICAL & ENGINEERING
™™™raMg SjEPTEMBER 26, |2qog -lEplTED BY|W||LLIAM G|. gCHMU I jkNET $. pqDP
MULTIPLE
BONDING
Cotton tells C&EN. T h e formulation of quadruple bonds in terms of the overlap of orbitals is clear, he says. But to consider a quintuple or sextuple bond "always entails some amount of heuristic argument." In theory, diatomic transition-metal species can form up to six bonds between their valence shell s and d orALBERT COTTON AND HIS This fivefold bond isn't as unbitals. A few sextuplecoworkers at Texas A&M expected as Cotton's first quadrubonded compounds • University surprised chemple bond some 4 0 years ago, but have been trapped at ists in 1964 with evidence that it still has chemists abuzz. low temperature and the {Re2Cl8}2~ ion contained the Characterization of fivefold observed spectroscopifirst known multiple bond bebonding in a stable compound "is cally, but they are untween two metal atoms. Not onan empirical landmark in the evolikely to be isolated as ly was it a multiple bond, it was an lution of our understanding of stable compounds at unprecedented quadruple bond. HIGH FIVE Quintuple metal-metal bonding in particuroom temperature. Cotton convinced the world that bond is proposed to form lar and bonding across the perihe was right, and inorganic chemby the sharing of five odic table in general," note Frank If ligands are used to istry hasn't been the same since. electron pairs in five A. Weinhold and Clark R. Landis stabilize the multiply bonding 3d orbitals in this of the University of Wisconsin, Now, Philip P. Power at the bonded metal atoms, chromium dimer with Madison. Weinhold and Landis University of California, Davis, the number of valence bulky terphenyl ligands have conducted extensive comand his coworkers report evidence orbitals available for (R = isopropyl). putational studies of transitionfor the first "quintuple" bond bemultiple bonding is remetal multiple bonding, and they tween two metal atoms in the duced. For quintuple agree that the Power group's dichromium complex, RCrCrR, bonds, each metal could structural, magnetic, and specwhere R is a bulky terphenyl lighave only one ligand, as is the case and {Science, published online troscopic data, supported by comin the chromium dimer. putational studies, "are consistent Sept. 2 2 , dx.doi.org/10.1126/sci Power's group is known for its ence.lll6789).The synthesis of compounds con0 with the formulation x of a quintuple bond." c h r o m i u m dimer taining multiple bonds between 2 "This new comexists as air- and heavier group 14 elements, such jjj pound fills a hole in moisture-sensitive as germanium-germanium triple ™ our understanding bonds. Key to the group's success dark red crystals | of the bonding be- has been designing bulky subthat are stable up to tween transition200 °C. stituents that stabilize molecules metal atoms," adds with atoms of low coordination Power and his Bjorn O. Roos of numbers, such as the 2,6-{(2,6coworkers believe Lund University, in diisopropyl)phenyl}phenyl group that the two chromiSweden, a theorist used with the chromium dimer. u m ® atoms, which who has investigated have a 3d5 electron The chromium dimer "is just a "I look forward multiple bonding in configuration, share start," Power says, and may not to seeing how transition metals and five electron pairs in Power be as important as what could reactinides. "The comfive bonding molecsult from the finding. "Hopefulthis story comes pound is certainly worthy of a ular orbitals. Power is cautious ly it will only be the forerunner of out as further more profound theoretical study/' about using the word "quintuple" other quintuple bonds." There are now thousands of to describe the bonding in the "I look forward to seeing how experiments examples of quadruple-bonded new molecule, preferring to call it this story comes out as further are done" transition-metal compounds, "fivefold bonding" because the experiments are done," Cotton including chromium(II) comactual bond order is likely less F, adds. "This is beautiful and fascipounds with quadruple bonds, than five. nating chemistry "—STEVE RITTER Texas A&M University
QUINTUPLE' BOND MAKES ITS DEBUT
First stable molecule with fivefold metal-metal bonding is synthesized
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26, 2005
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