Unusual binding properties, enabling the stabilization of elusive species, and beneficial properties for homogeneous catalysts have been predicted and demonstrated for ligand-stabilized main group fragments, such as carbodiphosphoranes and -carbenes. However, the quantification and comparison of their binding properties by experimental means still represent major challenges. In this article, we describe a series of iridium(III) pincer complexes of the type [(PEP)IrCl(CO)(H)](q) enabling the quantification of the donor strength of the central donor group E (q = 0, +1, +2). Our investigations show that phosphine-stabilized boron(I) and carbon(0) compounds are exceptionally strong neutral donor groups in comparison to common spectator ligands in homogeneous catalysis such as carbenes and phosphines. Our experimental and computational results for the first time allow and justify the comparison of the donor strength of cationic, neutral, and anionic ligands. On the basis of quantum chemical investigations, we further demonstrate that the heavier homologues of phosphine-stabilized borylenes and carbon(0) compounds exhibit slightly diminished donor properties.