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Blue and orange crystals revealed two distinct agostic forms of the same molybdenum complex. The 2013 discovery showed that changing which C–H bond interacts with a metal can alter a molecule’s electronic structure—but it did not demonstrate a better-performing catalyst.
What the researchers found
Edwin F. van der Eide, Ping Yang and R. Morris Bullock reported two agostic isomers of the same cation, [CpMo(CO)2(PiPr3)]+, paired with the weakly coordinating anion B(C6F5)4−. The forms were isolated as orange and blue crystals. Chemistry World described the work as the first experimental evidence for agostic isomers, also called agostomers, in organometallic complexes.
These were not two unrelated compounds. They shared the same constituent atoms and overall composition, but differed in the way a C–H bond interacted with the molybdenum center. The research paper reports that X-ray crystallography, spectroscopic methods and density functional theory (DFT) calculations were used to characterize the structures. The authors attributed the color difference to significantly different lowest unoccupied molecular orbital (LUMO) energies.
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What makes them agostic isomers?
An agostic interaction is a weak interaction between a C–H bond and a metal center. In an agostic isomer, the identity or arrangement of the C–H bond interacting with the metal differs from that in another form of the same complex. The bond is not simply an unrelated decoration: the interaction can affect the metal’s coordination environment and the molecule’s electronic structure.
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That distinction helps explain why the crystals could look different even though they contained the same cation. The structural assignment rested on crystallographic and spectroscopic evidence, supported by calculations—not on color alone. The paper connects the different agostic interactions with distinct LUMO energies, providing an electronic explanation for the blue-versus-orange appearance.
Why crystallization made the discovery possible
According to Chemistry World’s 15 August 2013 account, the team encountered the two colors while studying molybdenum complexes in a hydrogenation-catalyst context. The report says that separate agostic forms had been difficult to isolate because they interconverted rapidly in solution. Crystallization provided a way to capture and examine distinct forms in the solid state, where that switching was no longer an obstacle to structural characterization.
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The account identifies crystallization of an intermediate after phosphine abstraction as crucial. Both forms were present in significant amounts in solution, Bullock told Chemistry World, and each yielded single crystals suitable for diffraction. He cautioned that other agostic complexes may have multiple forms in solution even when only one form crystallizes.
What the discovery says—and does not say—about catalysis
The work arose in a catalytic context, but its direct result was structural: two isomers were isolated and characterized. Bullock explained that catalytic runs showed saturated, 18-electron species, while other evidence suggested that 16-electron species should participate in the catalytic cycle as fleeting intermediates. An agostic interaction can stabilize an unsaturated complex by temporarily engaging a C–H bond near a vacant metal site; that is a possible mechanistic role, not proof of improved catalytic performance.
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The reported crystal structures do not establish that either isomer makes hydrogenation faster, more selective, more economical or more practical. The cited paper and news account do not demonstrate industrial scale-up or a commercial application. The significance established here is that different agostic interactions can correspond to isolable structures with distinct electronic properties.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Publication details
The primary paper, “Isolation of Two Agostic Isomers of an Organometallic Cation: Different Structures and Colors,” appeared in Angewandte Chemie International Edition, volume 52, issue 39, pages 10190–10194 (2013). Wiley lists its first-publication date as 29 July 2013; PubMed lists the journal issue date as 23 September 2013.
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- Read the primary research paper.
- Read Chemistry World’s report.
- View the PubMed bibliographic record.
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