Evaluation of electronic and steric properties of new NHC ligands and catalysts. An indepth characterisation of the electronic properties of the three ligands was then carried out (Scheme 3 and Table ). The overall electron-donating ability of the NHC ligands was first quantified by recording the average stretching frequency, ν CO av , of the carbonyl ligands in complexes Rh1-Rh3, which is correlated to the Tolman Electronic Parameter (TEP) value of the involved ligand by a well-established linear correlation. Additionally, the σ-donating and the π-accepting abilities of the NHCs can be independently assessed by measuring the 1 J CH coupling constant between the carbon and the hydrogen atoms on the pre-carbenic position in the azolium precursors and by recording the chemical shift of 77 Se nuclei in the seleno adducts Se1-Se3 respectively. The experimental data first confirmed that the three benzimidazole-based NHCs are less donating than their imidazol(in)e-based congeners IMes and SIMes. Within the benzimidazolylidene series, the presence of the two distal phenyl groups in BI*Ph slightly increased its σ-donation but also its π-acidity relative to the unsubstituted BIPh, leading to a slightly stronger overall electronic donation with a TEP value of 2052.2 cm -1 . In contrast, perfluorination of the backbone-aryl groups induced a noticeable decrease in electronic donation with a TEP value ranging from 2052.2 cm -1 for BI*Ph to 2056.2 cm -1 for BIF*Ph, as could be anticipated in view of the strong electronwithdrawing effect of fluorine atoms. Overall, starting from the unsubstituted BIPh, the installation of the second aryl layer at the rear of the benzimidazolyl core generates the more electrondonating BI*Ph ligand and the less electron-donating BIF*Ph ligands. Scheme 3. A) Structures and abbreviations of NHC ligands studied herewith. B) Synthesis of rhodium carbonyl complexes and selenium adducts. Reaction conditions: a) i) 2.2 equiv. of benzimidazolium precursor, 2.2 equiv. KOC(CF 3 ) 2 CH 3 , THF, rt, 30 min., ii) 1 equiv. [RhCl(COD)] 2 , overnight; b) CO (excess), DCM, rt, 30 min.; c) i) 1 equiv. of benzimidazolium precursor, 1 equiv. KOC(CF 3 ) 2 CH 3 , THF, rt, 30 min., ii) 2 equiv. Se, overnight. [cm TEP value [cm -1 ]