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Although no example of bond stretch isomerism has been established in solution, two isomers have been crystallized for pentamethylcyclopentadienyl ruthenium dichloride dimer ( p*RuCl2sub>2). One has an Ru-Ru bond (2.93 Å) and the other has a long intermetallic distance of 3.75 Å. The former isomer is thought to be diamagnetic, and the latter is magnetic.{{cite journal, author=McGrady, John E., title= Cp*RuCl)2(μ-Cl)2 bond-stretch or spin-state isomerism?, journal=Angewandte Chemie International Edition, year=2000, volume=39, issue=17 , pages=3077–3079, doi=10.1002/1521-3773(20000901)39:17<3077::AID-ANIE3077>3.0.CO;2-B, pmid=11028037
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, 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.
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
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
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 In X-ray crystallography, crystallographic disorder describes the cocrystallization of more than one rotamer, conformer, or isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same numbe ...
was not modeled appropriately. Several such examples were uncovered.
Special examples
Bond stretch isomerism is confirmed for complexes subject tospin crossover
Spin crossover (SCO) is a phenomenon that occurs in some metal complexes wherein the spin state of the complex changes due to an external stimulus. The stimuli can include temperature or pressure. Spin crossover is sometimes referred to as spin ...
transitions. In some octahedral complexes of d6 configuration, the depopulation of eg orbitals causes significant contractions of the metal-ligand bond distances. The phenomenon is mainly manifested in the solid forms of the compounds.
Although no example of bond stretch isomerism has been established in solution, two isomers have been crystallized for pentamethylcyclopentadienyl ruthenium dichloride dimer ( p*RuCl2sub>2). One has an Ru-Ru bond (2.93 Å) and the other has a long intermetallic distance of 3.75 Å. The former isomer is thought to be diamagnetic, and the latter is magnetic.{{cite journal, author=McGrady, John E., title= Cp*RuCl)2(μ-Cl)2 bond-stretch or spin-state isomerism?, journal=Angewandte Chemie International Edition, year=2000, volume=39, issue=17 , pages=3077–3079, doi=10.1002/1521-3773(20000901)39:17<3077::AID-ANIE3077>3.0.CO;2-B, pmid=11028037
References