The electronic nature of Ni π‐complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π‐complexes, Ni(dtbpe)(X) (dtbpe=1,2‐bis(di‐tert‐butyl)phosphinoethane; X=alkene or carbonyl containing π‐ligands), is probed using a combination of 31P NMR, Ni K‐edge XAS, Ni Kβ XES, and DFT calculations. These complexes are best described as square planar d10 complexes with π‐backbonding acting as the dominant contributor to M−L bonding to the π‐ligand. The degree of backbonding correlates with 2JPP from NMR and the energy of the Ni 1s→4pz pre‐edge in the Ni K‐edge XAS data, and is determined by the energy of the π*ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π‐acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d10 electronic configuration. This ligand‐induced backbonding can be formally described as a 3‐centre‐4‐electron (3c‐4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ‐donors to the π*ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.

Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(μ-η(2):η(2)-C6H6)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η(2)-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl-S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η(2)-carbonyl complex, phenyl esters were found to predominantly undergo Caryl-O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean Caryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.

Reacting the low-valent nickel complex [(dtbpe)Ni](μ-η:η-CH) with oxaziridines was found to form mixtures of imine, amide and aldehyde products. If the N-substituent of the oxaziridine is sufficiently bulky, a short-lived intermediate can be isolated and characterized by X-ray diffraction studies as an oxazanickela(ii)cyclobutane. This is the first well-defined example of N-O oxidative addition of an oxaziridine to a transition metal. Subsequent fragmentation of this oxazanickelacyclobutane forms a complex mixture of products, including a nickel(ii) imido complex, demonstrating that oxaziridines can serve as nitrene precursors. Preliminary mechanistic analysis is consistent with a bimetallic mechanism of fragmentation of the oxazanickelacyclobutane to form the nickel imido and η-aldehyde complexes.

A mild protocol for Ni-catalyzed trifluoromethylthiolation of aryl chlorides and bromides is described herein. The method utilizes AgSCF as an easily accessible nucleophilic trifluoromethylthiolating reagent and does not require any ligands or additives. Ortho-selectivity is achieved using a variety of directing groups such as imines, pyridines, and oxazolines for 24 examples in up to 95% yield.