Bond stretch isomer
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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.
Special examples
[edit]Bond stretch isomerism is confirmed for complexes subject to spin crossover 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.
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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 ([Cp*RuCl2]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.[3]
References
[edit]- ^ Parkin, G., "Bond-Stretch Isomerism in Transition Metal Complexes: a Reevaluation of Crystallographic Data", Chem. Rev. 1993, volume 93, 887-911. doi:10.1021/cr00019a003
- ^ Gϋtlich, P. (2001). "Photoswitchable Coordination Compounds". Coordination Chemistry Reviews. 219–221: 839–879. doi:10.1016/S0010-8545(01)00381-2.
- ^ McGrady, John E. (2000). "[(Cp*RuCl)2(μ-Cl)2]: bond-stretch or spin-state isomerism?". Angewandte Chemie International Edition. 39 (17): 3077–3079. doi:10.1002/1521-3773(20000901)39:17<3077::AID-ANIE3077>3.0.CO;2-B. PMID 11028037.
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