Donor-Free Phosphenium–Metal(0)–Halides with Unsymmetrically Bridging Phosphenium Ligands

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 C_2v-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.