Trans-chelating diphosphines, the elusive ligands

The development of chelating diphosphine ligands has been a prolific research area for several decades, but quite surprisingly, from the plethora of diphosphines that are currently known, only a handful of them are able to coordinate preferentially in a trans-fashion (P-M-P angle close to 180°). The most representative ones, together with their most relevant reactivities and catalytic applications will be presented herein. Although some sophisticated backbones are currently known, a truly and only trans-coordinating diphosphine remains elusive. Apparently, the inherent flexibility associated with the required large backbones always left other coordination modes accessible. Cis-monodentates, dimeric species or bimetallic compounds are additional escape routes for the desired species. Nevertheless, on the way to this illusory goal compounds with surprising and fascinating reactivities and coordination modes have been encountered which surely have contributed to widen the knowledge of coordination chemistry.