Quantifying the donor strength of ligand-stabilized main group fragments

Unusual binding properties, enabling the stabilization of elusive species, and beneficial properties for homogenous catalysts have been predicted and demonstrated for ligand-stabilized main-group fragments, such as carbodiphosphoranes or -carbenes. However, the quantification and comparison of their binding properties by experimental means still represents a major chal-lenge. In the present manuscript, we describe a series of iridium(III) pincer complexes of the type [(PEP)Ir(H)Cl(CO)]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 like carbenes or phosphines. Our experimental and computational re-sults for the first time allow and justify the comparison of the donor strength of cationic, neutral and anionic ligands. Based on quantum chemical investigations, we fur...