NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

The chemistry of NHC core pincer ligands of LX(2) type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R(3)P(+)–CR(2)(−)) or by a heteroatom through amide (R(2)N(−)), oxide (RO(−)), or thio(seleno)oxide (RS(−), RSe(−)) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species.