Use of a Bidentate Ligand Featuring an N‑Heterocyclic Phosphenium Cation (NHP+) to Systematically Explore the Bonding of NHP+ Ligands with Nickel

A novel bidentate ligand featuring an N-heterocyclic phosphenium cation (NHP+) linked to a phosphine side arm is used to explore the coordination chemistry of NHP+ ligands with nickel. Direct P–Cl bond cleavage from a chlorophosphine precursor [PP]-Cl (1) by Ni­(COD)2 affords the asymmetric bimetallic complex [Cl2Ni­(μ-PP)2Ni] (2) via a nonoxidative process. Abstraction of the halide with either NaBPh4 or K­[B­(C6F5)4] prior to metal coordination to form the free phosphenium ligand [PP]+ in situ, followed by coordination to Ni­(COD)2, afforded the halide-free Ni0 complexes [(PP)­Ni­(COD)] [B­(C6F5)4] (4) and [(PP)­Ni­(COD)]­[BPh4] (5). Chloride abstraction from 1 is problematic in the presence of a PF6– counterion, however, as evident by the formation of [(PP)­Ni­(PP-F)]­[PF6] (3). The COD ligand in 5 can be readily displaced with PMe3 or PPh3 to afford [(PP)­NiL2]­[BPh4] (L = PMe3 (6), PPh3 (7)). Complexes 2–7 feature planar geometries about the NHP+ phosphorus atom and unusually short Ni–P distances, indicative of multiple bonding resulting from both P → Ni σ donation and Ni → P π backbonding. This bonding description is supported by theoretical studies using natural bond orbital analysis.