N-heterocyclic carbenes (NHCs) are playing an increasing role as ligands in organometallic catalysts. Their success as effective mediators of catalysis has generally been attributed to their strong sigma-donor ability, allowing for the formation of electron-rich metal centres and the stabilization of putative electron-poor intermediates. We have recently demonstrated that the electronic role of NHCs can be significantly more complex and are currently exploring the specific contributions to M-C bonding and how this affects chemical behaviour. We are particularly interested in defining the differences and similarities between metal-NHC and metal-phosphine bonds and how this impacts their performance as ligands in organometallic catalysis.