The incorporation of fluorine atoms into the N-heterocyclic carbene (NHC) ligand backbone in a Mn(I) complex [CpMn(CO)2(IMes)] was achieved via a sequence of the C–H bond deprotonation and electrophilic fluorination of the anionic abnormal carbene moiety with N-fluorobenzenesulfonimide. The demetallation of resulting complexes [CpMn(CO)2(IMesF1-2)] with triflic acid afforded the imidazolium salts IMesF1-2·HOTf, the precursors of the corresponding fluorinated NHCs. Interestingly, the IMesF2 ligand was found to be more electron-donating than its well-known analogue IMesCl2 notwithstanding the higher electronegativity of fluorine atoms. A systematic experimental and density functional theory study of this NHC series allowed us to fully rationalize this unexpected behavior, revealing that the weaker σ-donation of IMesF2 is compensated by its lower π-accepting properties due to the strong mesomeric +M effect of the fluorine substituents.