Understanding the mechanisms of catalytic transformations is extremely important as this allows chemists to improve reactions by, for example, reducing catalyst loading, preventing the formation of by-products, or obtaining fundamental knowledge about the structure and bonding of intermediates. The focus of this thesis lies with the recently developed transformations of Ni-catalysed C–O functionalisation and Ni-catalysed reductive coupling reactions. Although these reactions are facilitated by the favourable properties of Ni compared to Pd, the scope of possible mechanisms for a Ni-catalysed reaction is broader and less information is known about their mechanisms compared to Pd-catalysed transformations.

In this work, three mechanistic pictures for these Ni-catalysed transformations are presented. The first is a mechanistic study of the Ni-catalysed C–O silylation of aryl esters, where the first oxidative addition complexes of Ni and aryl esters with a monodentate phosphine are reported alongside the role of the fluoride-containing additives. The second is an investigation into a proposed intermediate in reductive coupling reactions – a rarely-isolated Ni(I)-alkyl complex. One example was isolated and characterised by EPR and X-ray crystallography. Significantly, its CO₂ insertion reactivity was also probed. The third project sheds light on the role of Zn as more than just a reductant in the Ni-catalysed carboxylation of arylsulfonium salts.