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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 phosphinestabilized 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.

We report a novel method for the preparation of PBP-pincer complexes from bis(phosphine)boronium salts. The central (RP)HB-moiety in a palladium complex is demonstrated to be a L-type ligand, therewith completing a series of pincer-type complexes with Z-, X- and L-type boron-based ligands, respectively.

The reactivity patterns of carbodiphosphoranes (CDPs) as ligands are much less explored than those of isoelectronic analogues. In the current manuscript, we investigate the reactivity of the carbodiphosphorane-based PCP nickel(ii) pincer complex [({dppm}2C)NiCl]Cl (1) towards acids and bases, calculate proton affinities, analyse the bonding situation and tautomeric forms with the aim to evaluate whether CDPs can potentially act as cooperative ligands in catalysis (dppm = 1,1-bis(diphenylphosphino)methane). Our investigations show that different tautomeric forms are stable for the coordinated and the uncoordinated ligand. The protonated CDP-based complex 2 represents a rare example of a cationic donor group binding to a cationic metal centre. The continuous arm-deprotonation of 1 leads to the formation of remarkably stable dimers with Ni-C-P-C-metallacycles. In comparison to corresponding boron and amine-based ligands, the coordinated CDP-group exhibits the lowest proton affinity according to DFT calculations, indicating that coordinated CDP ligands can potentially serve as proton relay in cooperative catalysis.

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