[Ni(NHC)₂ ] as a Scaffold for Structurally Characterized trans [H-Ni-PR₂ ] and trans [R₂ P-Ni-PR₂ ] Complexes

The addition of PPh₂ H, PPhMeH, PPhH₂ , P(para-Tol)H₂ , PMesH₂ and PH₃ to the two-coordinate Ni⁰ N-heterocyclic carbene species [Ni(NHC)₂ ] (NHC=IiPr₂ , IMe₄ , IEt₂ Me₂ ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR₂ ] or novel trans [R₂ P-Ni-PR₂ ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe₄ >IEt₂ Me₂ >IiPr₂ ) and phosphines are employed. P-P activation of the diphosphines R₂ P-PR₂ (R₂ =Ph₂ , PhMe) provides an alternative route to some of the [Ni(NHC)₂ (PR₂ )₂ ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)₂ (Ph₂ P-PPh₂ )] adducts proceeds with computed barriers below 10 kcal mol^-1 . The ability of the [Ni(NHC)₂ ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.