Biomass, especially lignin, is an attractive potential source for chemicals and fuels currently derived from petroleum. The valorisation of lignin is challenging due to its complex structure and the harsh conditions required for depolymerisation. Sub-nanometer clusters and atomically dispersed metal species are highly active catalysts, allowing for milder reaction conditions. This thesis focuses on the synthesis of organometallic complexes with N-heterocyclic carbene (NHC) ligands and their use as precursors for extremely active, highly dispersed metal catalysts. Chapter 1 will introduce biomass and lignin valorisation, lignin model compounds, sub-nanometric metal catalysts, and metal-NHC complexes. Chapter 2 will then explore the design and synthesis of ruthenium and nickel complexes, mostly novel, with N-heterocyclic carbene ligands bearing long alkyl wingtip chains. These complexes were designed to sterically prevent metal atom aggregation during reduction of the complexes. In addition, the activity of the synthesised Ru-NHC complexes towards homogeneous transfer hydrogenation of acetophenone is studied. In Chapter 3, nanoparticles and supported heterogeneous catalysts are generated from the metal-NHC complexes and applied to the C-O bond hydrogenolysis of diphenyl ether under mild conditions (140 °C, 15 bar H2). Initial catalytic screening selected one catalyst, Ru12b@MCM-41, as highly active for this transformation with good selectivity for the desired monocyclic products. Characterisation of this catalyst suggests that the active catalyst is a sub-nanometric Ru species. In Chapter 4, the catalytic activity of Ru12b@MCM-41 was further enhanced and determined to be more active than all literature reports found. Recycling and leaching tests indicated the catalyst's deactivation mechanism. Finally, Ru12b@MCM-41 was tested for the conversion of many substrates in a three-pronged scope. Overall, this thesis explores the use of designed organometallic complexes with specific properties as precursors for highly active, dispersed metal catalysts.