Mesoionic imidazolylidenes are recognized as excellent electron-donating ligands in organometallic and main group chemistry. However, these carbene ligands typically show poor π-accepting properties. We have performed a computational analysis of 71 mesoionic imidazolylidenes having different aryl or heteroaryl substituents in C2 position. The study has revealed that a diphenyltriazinyl (Dpt) substituent renders the corresponding carbene particularly π-acidic. The computational results could be corroborated experimentally. A mesoionic imidazolylidene with a Dpt substituent was found to be a better σ-donor and a better π-acceptor compared to an Arduengo-type N-heterocyclic carbene. To demonstrate the utility of the new carbene, we have used the ligand to stabilize a low-valent paramagnetic tin(II) compound.