Agostic Isomers: different colors and structures

Researchers from PNNL have, for the first time, isolated and characterized two agostic isomers (‘agostomers’). Isomers of a cationic Mo piano stool complex, important intermediates in catalytic hydrogenation, have been separated by crystallization and can interconvert upon dissolution. The γ-agostomer of [CpMo(CO)2(PiPr3)]+ has a blue color, while the β-agostomer is orange. ADF calculations confirm that the two agostomers are very close in energy and the calculated VIS/NIR spectra are in good agreement with the experimental data. While the frontier orbitals of the two isomers are very similar, the LUMO of the β-agostomer is higher in energy by 0.64 eV compared to the γ-agostomer, resulting in the observed higher absorption energy.

 

Agostomers orbitals DFT

LUMO and HOMO of β-agostomer (left) and γ-agostomer (right) of [CpMo(CO)2(PiPr3)]+. The β-agostomer absorbs shorter wavelength of light in the VIS absorption, resulting in orange-colored crystals for the β-agostomer and blue-colored crystals for the γ-agostomer.

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E. F. van der Eide, P. Yang, and R. M. Bullock, Isolation of Two Agostic Isomers of an Organometallic Cation: Different Structures and Colors, Angew. Chem. Int. Ed. 52, 10190-10194 (2013)

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