A convenient method of preparing two- and three-coordinate Ni(I) complexes of the form L-Ni^I-X (L = P ^t Bu₃, P ⁱ Pr₃, DPPE, NHC; X = -N(SiMe₃)(2,6- ⁱ Pr-C₆H₃), -O(2,6- ^t Bu₂-4-Me-C₆H₂)) is reported. Protonation of the easily prepared anionic Ni(I) bis(amido) complex K{Ni[N(SiMe₃)(2,6- ⁱ Pr-C₆H₃)]₂} in the presence of an appropriate L-type ligand results in loss of HN(SiMe₃)(2,6- ⁱ Pr-C₆H₃) and trapping of the resulting neutral Ni(I)-amido fragment to yield neutral, paramagnetic, two- and three-coordinate Ni(I) complexes. Protonation of these neutral amido complexes by the bulky phenol HO(2,6- ^t Bu₂-4-Me-C₆H₂) results in loss of the second amido moiety and trapping by the resulting phenoxide to yield Ni(I)-O(2,6- ^t Bu₂-4-Me-C₆H₂) complexes. The hapticity of the phenoxide ligand is influenced by the π-accepting ability of the L-type ligand. Where L = P ^t Bu₃, a poor π-acceptor, the phenoxide acts as a π-acceptor and adopts a η⁵-bonding mode through dearomatization of the phenyl ring. When L = NHC, a competent π-acceptor, the phenoxide acts as a π-donor, adopting a η¹-bonding mode through the O atom. The modular nature of this synthetic strategy allows variation of both the L- and X-type ligands of the complex in a stepwise fashion and should be extendable to a wide variety of ligand types for new Ni(I) complexes.

Recently, the development of more sustainable catalytic systems based on abundant first-row metals, especially nickel, for cross-coupling reactions has attracted significant interest. One of the key intermediates invoked in these reactions is a Ni(III) -alkyl species, but no such species that is part of a competent catalytic cycle has yet been isolated. Herein, we report a carbon-carbon cross-coupling system based on a two-coordinate Ni(II) -bis(amido) complex in which a Ni(III) -alkyl species can be isolated and fully characterized. This study details compelling experimental evidence of the role played by this Ni(III) -alkyl species as well as those of other key Ni(I) and Ni(II) intermediates. The catalytic cycle described herein is also one of the first examples of a two-coordinate complex that competently catalyzes an organic transformation, potentially leading to a new class of catalysts based on the unique ability of first-row transition metals to accommodate two-coordinate complexes.

The amido ligand -N(Si(i)Pr3)DIPP (DIPP = 2,6-diisopropylphenyl) has been used to prepare two-coordinate complexes of Cr(I), Cr(II), and Cr(III). The two-coordinate Cr(II) complex has also been used to prepare a three-coordinate Cr(III) iodide complex, which can be used to access a stable Cr(III) methyl species.

A cationic nickel complex of the bis(8-quinolyl)(3,5-di-tert-butylphenoxy)phosphine (NPN) ligand, [(NPN)NiCl]⁺, is a precursor to efficient catalysts for the hydrosilation of alkenes with a variety of hydrosilanes under mild conditions and low catalyst loadings. DFT studies reveal the presence of two coupled catalytic cycles based on [(NPN)NiH]⁺ and [(NPN)NiSiR₃]⁺ active species, with the latter being more efficient for producing the product. The preferred silyl-based catalysis is not due to a more facile insertion of alkene into the Ni-Si (vs. Ni-H) bond, but by consistent and efficient conversions of the hydride to the silyl complex.