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. Synthesis The synthesis of the ligands often involves the reaction between 1,3-dibromoethylbenzene with a secondary phosphine followed by deprotonation of the quaternary phosphorus intermediates to generate the ligand. To generate the metal complex, two common routes are employed. One is a simple oxidative addition of the ipso-C-X bond where X = Br, I to a metal centre, often a M(0) (M = Pd, Mo, Fe, Ru, Ni, Pt) though other metal complexes with higher oxidation states available can also be used (e.g. Rh(COD)Cl2). The other significant method of metal introduction is through C-H bond activation., The major difference is that the metal used in this method is already in a higher oxidation state (e.g. PdCl2 – Pd(II) species). However, these reactions have been found to proceed much more efficiently by employing metal complexes with weakly-bound ligands (e.g. Pd(BF4)2(CH3CN)2 or Pd(OTf)2(CH3CN)2 where OTf = F3CO2SO−). Role in catalysis The potential value of pincer ligands in catalysis has been investigated, although no process has been commercialized. Aspirational applications are motivated by the high thermal stability and rigidity. Disadvantages include the cost of the ligands. Suzuki-Miyaura coupling Pincer complexes have been shown to catalyse Suzuki-Miyaura coupling reactions, a versatile carbon-carbon bond forming reaction. Typical Suzuki coupling employ Pd(0) catalysts with monodentate tertiary phosphine ligands (e.g. Pd(PPh3)4). It is a very selective method to couple aryl substituents together, but requires elevated temperatures. Using PCP pincer-palladium catalysts, aryl-aryl couplings can be