Bromide abstraction from the three-coordinate Ni(i) ring-expanded N-heterocyclic carbene complex [Ni(6-Mes)(PPh)Br] (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) with TlPF in THF yields the T-shaped cationic solvent complex, [Ni(6-Mes)(PPh)(THF)][PF] (2), whereas treatment with NaBAr in EtO affords the dimeric Ni(i) product, [{Ni(6-Mes)(PPh)}(μ-Br)][BAr] (3). Both 2 and 3 act as latent sources of the cation [Ni(6-Mes)(PPh)], which can be trapped by CO to give [Ni(6-Mes)(PPh)(CO)] (5). Addition of [(EtSi)(μ-H)][B(CF)] to 1 followed by work up in toluene results in the elimination of phosphine as well as halide to afford a co-crystallised mixture of [Ni(6-Mes)(η-CHMe)][B(CF)] (4), and [6MesHCHMe][B(CF)]. Treatment of 1 with sodium salts of more strongly coordinating anions leads to substitution products. Thus, NaBH yields the neutral, diamagnetic dimer [{Ni(6-Mes)}(BH)] (6), whereas NaBH(CN) gives the paramagnetic monomeric cyanotrihydroborate complex [Ni(6-Mes)(PPh)(NCBH)] (7). Treatment of 1 with NaOBu/NHPh affords the three-coordinate Ni(i) amido species, [Ni(6-Mes)(PPh)(NPh)] (8). The electronic structures of 2, 5, 7 and 8 have been analysed in comparison to that of previously reported 1 using a combination of EPR spectroscopy and density functional theory.

We report a new series of homoleptic Ni(I) bis-Nheterocyclic carbene complexes with a range of torsion angles between the two ligands from 68°to 90°. Electron paramagnetic resonance measurements revealed a strongly anisotropic g-tensor in all complexes with a small variation in g ∥ ∼ 5.7−5.9 and g ⊥ ∼ 0.6. The energy of the first excited state identified by variable-field far-infrared magnetic spectroscopy and SOC-CASSCF/NEVPT2 calculations is in the range 270−650 cm −1 . Magnetic relaxation measured by alternating current susceptibility up to 10 K is dominated by Raman and direct processes. Ab initio ligand-field analysis reveals that a torsion angle of <90°causes the splitting between doubly occupied d xz and d yz orbitals, which has little effect on the magnetic properties, while the temperature dependence of the magnetic relaxation appears to have no correlation with the torsion angle.

Studies on alkyne hydrophosphination employing nickel‐NHC catalysts (NHC=N‐heterocyclic carbene) revealed that the free N‐alkyl substituted NHCs themselves were catalytically active. DFT calculations showed the mechanism involves the NHC acting as a Brønsted base to form an imidazolium phosphide species which then undergoes rate‐limiting nucleophilic attack at the terminal alkyne carbon. This mechanism explains the preference seen experimentally for reactions with aryl substituted phosphines and alkynes, while the rearrangements of the alkenyl anion formed upon P−C bond formation account for the observation of both Z‐ and E‐regioisomers of the products.

The addition of PPh 2 H, PPhMeH, PPhH 2 , P(para-Tol)H 2 , PMesH 2 and PH 3 to the two-coordinate Ni 0 Nheterocyclic carbene species [Ni(NHC) 2 ] (NHC = IiPr 2 , IMe 4 , IEt 2 Me 2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [HÀ NiÀ PR 2 ] or novel trans [R 2 PÀ NiÀ PR 2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe 4 > IEt 2 Me 2 > IiPr 2 ) and phosphines are employed. PÀ P activation of the diphosphines R 2 PÀ PR 2 (R 2 = Ph 2 , PhMe) provides an alternative route to some of the [Ni(NHC) 2 (PR 2 ) 2 ] complexes.DFT calculations capture these trends with PÀ H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the NiÀ P bond. PÀ P bond activation from [Ni(NHC) 2 (Ph 2 PÀ PPh 2 )] adducts proceeds with computed barriers below 10 kcal mol À 1 . The ability of the [Ni(NHC) 2 ] moiety to afford isolable terminal phosphido products reflects the stability of the NiÀ NHC bond that prevents ligand dissociation and onward reaction.