Bond stretch isomer

In chemistry, bond stretch isomerism is a concept of isomerism based on variations of bond length. The concept was proposed in the 1970s but was refuted in the 1990s.^[1]

The phenomenon was first invoked to explain the observation of blue and green isomers of mer-MoOCl₂(PMe₂Ph)₃, where PMe₂Ph is dimethylphenylphosphine. These isomers were shown, purportedly, by X-ray crystallography to differ with respect to the length of the Mo-O bond, which differed by 0.2 Å. Subsequent work showed that the supposed green bond stretch isomer consisted of blue mer-MoOCl₂(PMe₂Ph)₃ contaminated with a small amount of yellow mer-MoCl₃(PMe₂Ph)₃. The nearly isomorphous replacement of Mo-O unit with small amounts of Mo-Cl unit results in artifactually long Mo-O distance in the green sample. In essence the deception arises because the crystallographic disorder was not modeled appropriately. Several such examples were uncovered.