chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, bond stretch isomerism is a concept of

isomerism In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

based on variations of

bond length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...

. 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 , where is

dimethylphenylphosphine Dimethylphenylphosphine is an organophosphorus compound with a formula P(C6H5)(CH3)2. The phosphorus is connected to a phenyl group and two methyl groups, making it the simplest aromatic alkylphosphine. It is colorless air sensitive liquid. It i ...

. These isomers were shown, purportedly, by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

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 contaminated with a small amount of yellow . The nearly

isomorphous In mathematics, an isomorphism is a structure-preserving mapping or morphism between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. ...

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 where the center of mass of each form is identical or unresolvable. Due to disorder, the crystallographic solution ...

was not modeled appropriately. Several such examples were uncovered.

Special examples

Bond stretch isomerism is confirmed for complexes subject to

spin crossover Spin crossover (SCO) is a phenomenon that occurs in some metal Coordination complex, complexes wherein the Spin states (d electrons), spin state of the complex changes due to an external stimulus. The stimuli can include temperature, pressure or ...

transitions. In some octahedral complexes of d⁶ configuration, the depopulation of e_g 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 Pentamethylcyclopentadienyl ruthenium dichloride is an organoruthenium chemistry with the formula C5(CH3)5)RuCl2, commonly abbreviated p*RuCl2. This brown paramagnetic solid is a reagent in organometallic chemistry. It is an unusual examp ...

( 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.

References

Isomerism {{Stereochemistry-stub