N‑Heterocyclic Carbene-Phosphinidene and Carbene-Phosphinidenide Transition Metal Complexes

Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)­PH] (1, IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl2]2, which afforded the three-legged piano-stool complexes [LMCl2{HP­(IPr)}] (9a/9b: M = Ru/Os, L = η6-p-cymene; 10a/10b: M = Rh/Ir, L = η5-C5Me5). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl­{P­(IPr)}] (11a/11b: M = Ru/Os; 12a/12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)­PTMS] (2) with [LMCl2]2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me3SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, which can be ascribed to the ability of the imidazole moiety to effectively stabilize a positive charge. Dehydrohalogenation of complexes 9/10 to 11/12 affords a significant increase of the metal-phosphorus bond order, with the carbene-phosphinidenide ligand acting as a strong 2σ,2π-electron donor.