Bond‐Stretch Isomers: Fact or Artifact?

It's almost a question of faith: Do bond‐stretch isomers exist? This apparent phenomenon first came to light in 1971 when results of X‐ray structural analyses of octahedral molybdenum complexes revealed that compounds with the same arrangement of ligands could differ only, but significantly, in the length of the MO bond. After this and other examples were explained by theory, it seemed that nothing could prevent the final acceptance of the concept. But later experimental studies proved that the different bond lengths were actually an artifact caused by impurities in the crystal. The recent report of the synthesis and characterization of two forms of [Nb(E)Cl3(PMe3)3] (E = O, S) by Gibson, McPartlin et al. (Angewandte Chem. Int. Ed. Engl. 1991, 104, 980), however, appears to have started the debate anew.