Metal-Oxo Papers Retracted

Emory University chemist Craig L. Hill and a contingent of international colleagues have retracted three research papers after they discovered their structural interpretations of transition-metal-oxo complexes were incorrect. The team recently reported the correct formulations (Inorg. Chem., DOI: 10.1021/ic2008914).

In the original studies, Hill and coworkers described the synthesis and characterization of platinum, palladium, and gold complexes in which one of the ligands is a single oxygen atom multiply bonded to the central metal (C&EN, Sept. 17, 2007, page 32). Such species could be important in catalytic technologies as varied as automobile catalytic converters, fuel cells, and industrial oxidation reactions.

But the existence of these complexes was born in controversy. Terminal metal-oxo complexes are common for early- and middle-transition metals––those in periodic table groups 3 to 8. However, in moving from left to right across the periodic table, transition-metal d orbitals fill up with electrons, reducing the ability of the metal to accept electrons donated by an oxo ligand. As a result, metal-oxo complexes in higher groups become progressively less stable.

Conventional wisdom has held that an “oxo wall” exists along the boundary between groups 8 and 9: Group 9 terminal metal-oxo complexes are rare, and for groups 10 to12—containing palladium, platinum, and gold—they are improbable and only one example now possibly exists (C&EN, Oct. 27, 2008, page 10).

The concept of the oxo wall is so entrenched that inorganic chemists demand extraordinary evidence of any complex that might breach it. Hill and coworkers believed they had provided such proof when they reported a terminal platinum-oxo complex in 2004 (Science, DOI: 10.1126/science.1104696). A year later, Hill’s group repeated the feat by preparing a terminal palladium-oxo complex (J. Am. Chem. Soc., DOI: 10.1021/ja054131h), followed by a terminal gold-oxo complex in 2007 (J. Am. Chem. Soc., DOI: 10.1021/ja072456n). It turns out that Hill and his colleagues were wrong.

“We were always skeptical about our interpretation of the data and formulation of these complexes,” Hill tells C&EN. His group continued its experiments and eventually found evidence that a tungsten atom is at the heart of two of the butterfly-shaped polytungstate complexes, not platinum or gold as originally reported. For the third complex, a palladium and a tungsten atom share the duty of connecting two polytungstate ligands.

“We thought it best to retract the papers,” Hill notes. “All the data in the three papers are correct—no one has challenged the experiments—but our interpretation was wrong. Thankfully, it is us correcting the work, and not someone else.” The three papers remain available to the chemistry community; two of the papers have been officially retracted and the Science retraction is pending. The two retraced papers now have an appended message stating that the publications have been withdrawn.

Like most inorganic chemists viewing the original papers, Paul R. Sharp of the University of Missouri, Columbia, was skeptical of the results. One could not argue against what the data were showing, Sharp says. Yet, the original formulations could not be modeled computationally—the complexes violated basic transition-metal bonding principles, he notes.

“Crystal structures can fool you,” Sharp continues. “What is remarkable in this case is that all the other data collected on the samples could be interpreted to match the erroneous formulations. Hill and his team should be applauded for continuing to investigate and for finally uncovering the correct formulations.”