The work reported in this thesis is concerned with the preparation, metal coordination and catalytic applications of expanded eight-membered N-heterocyclic carbenes. This is divided into four chapters, which cover the following areas of research. Chapter One provides the historical and literature overview of synthesis, reactions and catalysis applications for the six- and seven-membered NHCs. Structure and electronic properties of the expanded ring NHC systems are also discussed and compared with their phosphine and five-membered NHC counterparts. Chapter Two focuses on the synthesis and characterisation of symmetrical, unsymmetrical and backbone functionalised eight-membered NHC halide salts and free carbenes. The precursor salts were all prepared by the amidine route while treatment with KHMDS forms the free NHCs. The 8-NHC ring adopts a boat conformation with the backbone folded over the central NHC carbon. Their N-CNHC-N angles are large, ranging between 129° and 131°, with the attendant CNHC-N-CN-substituent angles being very small. Chapter Three describes the synthesis and characterisation of a series of Ag(I), Rh(I), Ir(I) and Ni(I) complexes. Structural analysis of silver-(8-NHC) complexes reveal that these large rings cause the aromatic substituents on the ring nitrogens to bend around and essentially enclose the Ag centre. The percentage buried volume (%Vbur) of silver complexes increases considerably as the ring size expands from 5- to 8-membered NHC, reflecting the high steric demand imposed by the larger ring systems. It was also found that the 8-Mes ligand was too large to coordinate to the [Rh/IrCl(COD)]2 dimer, only the less sterically demanding 8-o-Tol was able to form stable [Rh/IrCl(8-o-Tol)(COD)] complexes. Comparison of CO stretching frequencies for [RhCl(NHC)(CO)2] complexes with 5-, 6-, 7- and 8-membered rings reveal that 8-o-Tol is the most basic of all the NHC ligands. A series of paramagnetic (8-NHC) Ni(I) complexes were prepared also, along with their characterization and EPR analysis. Chapter Four provides the results of catalytic performances for the 8-membered NHC rhodium and iridium complexes in the transfer hydrogenation of ketones. 8-NHC nickel complexes were also tested as catalysts in Suzuki and Kumada cross-coupling reactions.