sigma-Acceptor, Z-type ligands for transition metals

The ability of Lewis acids to coordinate to transition metals as sigma-acceptor ligands was recognized as early as in the 1970's, but so-called Z-type ligands remained curiosities until the early 2000's. Over the last decade, significant progress has been made in this area, especially via the incorporation of Lewis acid moieties into multidentate, ambiphilic ligands. Our understanding of the nature and influence of TM -> Z interactions has considerably improved and the scope of Lewis acids susceptible to behave as sigma-acceptor ligands has been significantly extended. This feature article summarizes these recent achievements.