ADAPTED FROM SCIENCE

Yet that is exactly what chemists in Japan now claim: the first isolation and structural characterization of two formally hexavalent palladium complexes [Science, 295, 308 (2002)]. If the discovery is confirmed, it will be one for the textbooks.

Palladium--which forms exceptionally efficient catalysts for organic transformations-- generally prefers lower oxidation states such as 0, II, and even IV. Transition-metal compounds with the highest oxidation states usually are obtained using highly electronegative ligands such as fluorine and oxygen. For example, PtF₆ and IrF₆ are known, but the analogous compound, PdF₆, apparently has yet to be confirmed. And no one has presented definitive evidence for an organometallic Pd(VI) species, Crabtree points out in a commentary accompanying the Japanese report.

The new formal Pd(VI) complexes are described by Wanzhi Chen, Shigeru Shimada, and Masato Tanaka of the National Institute of Advanced Industrial Science & Technology (AIST), in Tsukuba. In these complexes, the Pd(VI) center is surrounded by six silicon atoms--a far cry from electronegative ligands like fluorine. These complexes are "organometallic, at least in spirit, because silicon is the nearest chemical relative of carbon," Crabtree writes.

The complexes, which contain two Pd(II) centers in addition to Pd(VI), were prepared unexpectedly by the condensation reaction of three molecules of a simple Pd(II) complex bearing a bidentate silyl ligand.

X-ray crystal structures of the trinuclear complexes reveal that, in two pairs of silicon atoms, the Si–Si distance is short enough to allow some bonding interaction. If indeed there is some kind of Si–Si bond between these atomic neighbors, the complex may more accurately be described as a bis s-complex of Pd(II) in which the two Si–Si bonds coordinate to the palladium. This bonding situation, though not involving Pd(VI), is "equally unprecedented," the AIST researchers note. Further study will be necessary to clarify which bonding model is most appropriate, they add.

Crabtree suggests that changing the groups on the silicon atoms "should push the compound toward one or the other limiting structure."

The discovery "not only opens up what may be a wide field of such high-valent organometallic compounds and of Si–Si complexes but allows us to consider Pd(VI) and Si–Si complexes as intermediates in certain catalytic reaction mechanisms," Crabtree concludes.

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