Mononuclear palladium(I) and palladium(III) coordination compounds

Palladium coordination complexes are invaluable catalysts in organometallic reactions, facilitating a plethora of synthetically useful organic transformations that include C–H functionalization and C–C/C–heteroatom bond formation reactions. The proposed mechanisms for such reactions usually invoke two-electron pathways involving diamagnetic Pd⁰, Pd^II, and Pd^IV intermediates. However, recent research has focused on the viability of paramagnetic Pd species with +1 and +3 oxidation states as plausible intermediates. The past two decades have seen a renewed interest in the isolation and characterization of such transient species to obtain a better understanding of their structure and reactivity. This review focuses on the coordination chemistry of mononuclear Pd^I and Pd^III compounds that have been isolated and characterized using spectroscopic techniques such as electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), cyclic voltammetry (CV), electrospray ionization mass spectrometry (ESI-MS), single crystal X-ray crystallography (SC-XRD), and X-ray absorption spectroscopy (XAS) methods. It is expected that the knowledge gained from studying the electronic structure and spectroscopic properties of these compounds will be used to facilitate new modes of reactivity inaccessible to traditional Pd^0/II/IV chemistry.