Reactions of (cod)MCl2 (cod = 1,5 cyclooctadiene, M = Pd, Pt) with N-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)MCl]2 (NHP = N-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd2(dba)3. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain μ-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual C2v-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium-metal(0)-halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of σ-donor/π-acceptor contributions in the two M-P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)-halide fragments.

The outcome of the reduction of [(cod)PtX] (X = Cl, Br; cod = 1,5-cyclooctadiene) with N-heterocyclic phosphenium hydrides NHP-H depends strongly on the steric demand of the N-aryl group R and the nature of X. Reaction of [(cod)PtCl] with NHP-H featuring bulky N-Dipp groups produced an unprecedented monomeric phosphenium metal(0) halide [(NHP)(NHP-H)PtCl] stabilized by a single phosphine ligand. The phosphenium unit exhibits a pyramidal coordination geometry at the phosphorus atom and may according to DFT calculations be classified as a Z-type ligand. In contrast, reaction of [(cod)PtBr] with the sterically less protected NHP-H afforded a mixture of donor-ligand free oligonuclear complexes [{(NHP)PtBr}] (n = 2, 3), which are structural analogues of known palladium complexes with μ-bridging phosphenium units. All reductions studied proceed via spectroscopically detectable intermediates, several of which could be unambiguously identified by means of multinuclear (H, P,Pt) NMR spectroscopy and computational studies. The experimental findings reveal that the phosphenium hydrides in these multistep processes adopt a dual function as ligands and hydride transfer reagents. The preference for the observed intricate pathways over seemingly simpler ligand exchange processes is presumably due to kinetic reasons. The attempt to exchange the bulky phosphine ligand in [(NHP)(NHP-H)PtCl] by MeP resulted in an unexpected isomerization to a platinum(0) chlorophosphine complex via a formal chloride migration from platinum to phosphorus, which accentuates the electrophilic nature of the phosphenium ligand. Phosphenium metal(0) halides of platinum further show a surprising thermal stability, whereas the palladium complexes easily disintegrate upon gentle heating in dimethyl sulfoxide to yield metal nanoparticles, which were characterized by TEM and XRD studies.

The reaction of symmetrical bis(diazaphospholenyl) compounds with [MCl2(cod)] (M = Pd, Pt; cod = 1,5‐cyclooctadiene) has been used to prepare N‐heterocyclic phosphenium (NHP) metal(0) halides [M(NHP)Cl]n with diverse N substituents. Characterisation by ESI‐MS and NMR spectroscopy revealed that N‐tBu‐ and N‐aryl‐substituted NHP precursors yield trimeric (n = 3) and dimeric (n = 2) products, respectively; the degree of aggregation is presumably controlled by the different steric requirements of the NHP moiety. A single‐crystal XRD study of a trimeric Pd complex confirmed the proposed constitution of the complexes with μ2‐bridging NHP and terminal chlorido ligands and allowed their metrical parameters to be analysed for the first time undisturbed by crystallographic disorder. Studies of ligand substitution and redistribution processes revealed that the complexes withstand exchange of the μ‐bridging NHP units but tolerate substitution of the terminal chlorido ligands by other strong nucleophiles (SCN–, I–) with conservation of the oligomeric framework. Attempts to replace chlorido by thiolato ligands led to the discovery of a reaction between a phosphenium metal thiolate [Pd(NHP)(SR)]3 and CH2Cl2 to give a formaldehyde dithioacetal H2C(SR)2 (R = benzyl). This reaction could be developed into a protocol for a C–S cross‐coupling reaction in the presence of a catalytic amount of the phosphenium complex.

Abstract. 2-Bromo-and 2-chloro-1,3,2-diazaphospholenes react with (tht)AuCl to afford isolable N-heterocyclic phosphenium (NHP) dihalido-aurates, which were characterized by analytical and spectroscopic data and in one case by a single-crystal X-ray diffraction study. The T-shaped metal coordination sphere found in the crystal consists of a pseudo-linear AuX 2 unit that is perturbed by a weakly bound NHP unit. DFT studies indicate that the subunits interact mainly through electrostatic and dispersion forces, with negligible covalent contributions, and that the phosphenium dibromido-aurate is slightly more