Nickel and Palladium Catalysis: Stronger Demand than Ever

Key similarities and differences of Pd and Ni in catalytic systems are discussed. Overall, Ni and Pd catalyze a vast number of similar C–C and C–heteroatom bond-forming reactions. However, the smaller atomic radius and lower electronegativity of Ni, as well as the more negative redox potentials of low-valent Ni species, often provide higher reactivity of Ni systems in oxidative addition or insertion reactions and higher persistence of alkyl-Ni intermediates against β-hydrogen elimination, thus enabling activation of more reluctant electrophiles, including alkyl electrophiles. Another key point relates to the higher stability of the open-shell electronic configurations of Ni(I) and Ni(III) compared with Pd(I) and Pd(III). Nickel systems very often involve a number of interconvertible Ni(n+) active species of variable oxidation states (Ni(0), Ni(I), Ni(II), and Ni(III)). In contrast, catalytic reactions involving Pd(I) or Pd(III) active species are still relatively less developed and may require facilitation by special ligands or merging with photo- or electrocatalysis. However, the relatively high redox potentials of Pd(n+) species ensure their facile reduction to Pd(0) species under the assistance of numerous reagents or solvents, providing relatively high concentrations of molecular Pd1(0) complexes that can reversibly aggregate into active Pdn clusters and nanoparticles to form a cocktail of interconvertible Pdn(0) active species of various nuclearities (i.e., various values of "n"). Nickel systems involving Ni(0) complexes often require special strong reductants; they are more sensitive to deactivation by air and other oxidizers and, as consequence, often operate at higher catalyst loadings than palladium systems in the same reactions. The ease of activation and relatively high stability of low-valent active Pd species provide high robustness and versatility for palladium catalysis, whereas a variety of Ni oxidation states enables more diverse and uncommon reactivity, albeit requiring higher efforts in the activation and stabilization of nickel catalytic systems. As a point for discussion, we may note that Pd catalytic systems may easily form a "cocktail of particles" of different nuclearities but similar oxidation states (Pd1, Pdn, Pd NPs), whereas nickel may behave as a "cocktail of species" in different oxidation states but is less variable in stable nuclearities. Undoubtedly, there is stronger demand than ever not only to develop improved efficient catalysts but also to understand the mechanisms of Pd and Ni catalytic systems.