Catalyst tuning by changing ligands is a well-established protocol in transition-metal chemistry. N-Heterocyclic carbenes (NHCs) and tertiary phosphines (R3P) are the ubiquitous ligand actors. Here we demonstrate that the relative sigma-donor/pi-acceptor ability of the NHC ligand itself can be influenced by a simple substituent-controlled conformational. change, thereby directly impacting the reactivity of the transition-metal complex.