Classical carbenes are highly reactive species that have traditionally been regarded as transient intermediates, making direct structural characterization a challenge. Among them, N-heterocyclic carbenes (NHCs) stand out for their stability and broad applicability, yet atomic-scale insights into their structure and electronic properties remain limited. Here, we report the on-surface single-molecule characterization of a free NHC deposited on ultrathin insulating NaCl layers on Au(111), enabling direct investigation of a reactive carbene in its free form. Using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT), we resolve its molecular structure and frontier orbital with sub-molecular resolution. We further demonstrate the reactivity of the free NHC through its on-surface complexation with a gold atom. These results demonstrate that real-space techniques enable direct insights into the structure and reactivity of free NHCs, helping to connect molecular properties with their broader functional applications.