The mono-(2) and bis(amino-NHC) (3) nickel halide complexes can be prepared in moderate yield by reacting Ni(PPh 3 ) 2 X 2 with the amino pendant linked NHC 1, [C{(MES)N(CHCH)-N(CH 2 CH 2 NHtBu)}], or its lithium bromide carbene adduct 1-LiBr. The mono(amino-carbene) nickel complexes (2) are paramagnetic species bearing a tetrahedrally distorted Ni center featuring a bidentate coordination mode via NHC and amine dative bonds. In contrast, the bis(amino-carbene) nickel complexes (3) are proton NMR active, consisting of a square-planar geometry with two trans NHC moieties. Both complexes 2 and 3 contain nickel-carbene bonds that are comparable to the literature value. Two forms of isomeric products for 3 are observed in the solution structure, but only solid structural syn-trans isomers are obtained, despite numerous attempts using various solvents with different polarity. On the basis of the powder X-ray analysis and various spectroscopy and theoretical results, we have ruled out a cis-trans isomerization, but confirmed a syn-anti conformational equilibrium exhibited in the solution of 3. The variable-temperature NMR for complexes 3 are consistent with restricted rotation about the nickel-carbene carbon bond. Plots of K eq vs 1/T are obtained with calculated ΔH°of 6.03 and 7.61 kJ/mol and ΔS°of 16.1 and 24.1 J/mol 3 K in toluene-d 8 for 3-Br and 3-Cl, respectively, indicating that syn-anti conformers of 3 are more stable. The outcome of the theoretical calculation is in line with the experimental result.

We have obtained boronium complexes upon reaction of an amino-NHC lithium bromide adduct with borane. Unexpectedly, the treatment of the boronium complexes with silver triflate led to the isolation of a bis-borane compound containing two chemically different boron centers. The bisborane and boronium complexes can interconvert via a fourmembered intermediate metallacycle consisting of a three-center, two-electron B-H-B bonding motif assisted by the pendant amine arm.