The pincer ligand is most often an anionic, two-electron donor to the metal centre. It consists of a rigid, planar backbone usually consisting of aryl frameworks and has two neutral, two-electron donor groups at the meta-positions. The general formula for pincer ligands is 2,6-(ER)CH – abbreviated ECE – where E is the two-electron donor and C is the ipso-carbon of the aromatic backbone (e.g. PCP – two phosphine donors). Due to the firm tridentate coordination mode, it allows the metal complexes to exhibit high thermal stability as well as air-stability. It also implies that a reduced number of coordination sites are available for reactivity, which often limits the number of undesirable products formed in the reaction due to ligand exchange, as this process is suppressed.
There are various types of pincer ligands that are used in transition metal catalysis. Often, they have the same two-electron donor flanking the metal centre, but this is not a requirement.
The most common pincer ligand designs are PCP, NCN, PCN, SCS, and PNO. Other elements that have been employed at different positions in the ligand are boron, arsenic, silicon, and even selenium.
By altering the properties of the pincer ligands, it is possible to significantly alter the chemistry at the metal centre. Changing the hardness/softness of the donor, using electron-withdrawing groups (EWGs) in the backbone, and the altering the steric constraints of the ligands are all methods used to tune the reactivity at the metal centre.