Towards Hexagonal Planar Nickel: A Dispersion-Stabilised Tri-Lithium Nickelate

Advancing the understanding of lithum nickelate complexes, here we report a family of homoleptic organonickelate complexes obtained by reacting Ni(COD)₂ and lithium aryl-acetylides in the presence of the bidentate donor TMEDA. These compounds represent rare examples of low-valent transition-metals supported solely by organolithium ligands. Whilst the solid-state structures indicate a hexagonal planar geometry around Ni⁰ with Ni-Li bonds, bonding analysis via QTAIM, NCI, NBO and ELI methods reveals that the Ni-Li interactions are repulsive in nature, characterising these complexes as tri-coordinated. London dispersion forces between TMEDA and the organic substituents on nickel are found to play a crucial role in the stabilisation and thus isolation of these complexes. Preliminary reactivity studies demonstrate that the homoleptic lithium nickelates undergo stoichiometric cross-coupling with PhI to give dinickel clusters containing both anionic acetylide and neutral alkyne ligands.

Redox-active ligands and organic radical chemistry

Knowledge about bonding in diiminepyridine (L) halide, alkyl, and dinitrogen complexes of the metals iron, cobalt, and nickel is summarized, and two new examples are added to the set: L(1)Ni(Me) and L(1)Ni(N(2)). Reactivity of these types of complexes is discussed in terms of organic radical chemistry. New C-C couplings with L(2)CoAr complexes are described and proposed to involve halide abstraction and radical coupling. Calculations support the high tendency of the diiminepyridine ligand to accept an electron coming from a metal-carbon bond and so facilitate loss of a radical.

A dinuclear nickel(I) dinitrogen complex and its reduction in single-electron steps

Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).