<jats:title>Abstract</jats:title><jats:p>The electron‐donating properties of N‐heterocyclic carbenes ([N,N′‐bis(2,6‐dimethylphenyl)imidazol]‐2‐ylidene and the respective dihydro ligands) with 4,4′‐R‐substituted aryl rings (4,4′‐R=NEt<jats:sub>2</jats:sub>, OC<jats:sub>12</jats:sub>H<jats:sub>25</jats:sub>, Me, H, Br, S(4‐tolyl), SO(4‐tolyl), SO<jats:sub>2</jats:sub>(4‐tolyl)) were studied. Twelve new N‐heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)<jats:sub>2</jats:sub>(NHC)]. Cyclic voltammetry (Δ<jats:italic>E</jats:italic><jats:sub>1/2</jats:sub>) and IR (<jats:styled-content>$\tilde \nu $<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-ueqn-1.gif" xlink:title="equation image" /></jats:styled-content>(CO)) can be used to measure the electron‐donating properties of the carbene ligands. Modifying the 4‐positions with electron‐withdrawing substituents (4‐R=−SO<jats:sub>2</jats:sub>Ar, Δ<jats:italic>E</jats:italic><jats:sub>1/2</jats:sub>=+0.92 V) results in NHC ligands with virtually the same electron‐donating capacity as a trialkylphosphine in [IrCl(cod)(PCy<jats:sub>3</jats:sub>)] (Δ<jats:italic>E</jats:italic><jats:sub>1/2</jats:sub> =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4‐R=NEt<jats:sub>2</jats:sub> (Δ<jats:italic>E</jats:italic><jats:sub>1/2</jats:sub>= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.</jats:p>