Bond-stretch isomerism in strained inorganic molecules and in transition metal complexes:: A revival?

A decade after Parkin's statement that 'there is presently no definitive evidence of bond-stretch isomerism in (transition metal) complexes', bond-stretch isomerism (BSI) is revisited, with special emphasis on inorganic and organometallic chemistry. It appears that the characterization of real bond-stretch isomers, distinct, separable and stable at room temperature, remains a very rare event. In fact, the vast majority of reports point out the case of neighboring species that were characterized with structurally distinct cores or frameworks, depending on the electronic or steric influence of peripheral substituents. In such cases, the characterization of separate energy minima on the ground state potential energy surface of an isolated molecule provides a criterion for proving the relevance of the BSI concept and also a guideline toward the synthesis of real bond-stretch isomers by means of a proper tuning of the substituents. Functionalized polyoxometalates and complexes stabilizing the (Cr-3)(6+) linear framework might provide illustrations of this strategy in the near future.