The Importance of Ligand‐Induced Backdonation in the Stabilization of Square Planar d¹⁰ Nickel π‐Complexes

Abstract

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.

Aldehydes and ketones influence reactivity and selectivity in nickel-catalysed Suzuki–Miyaura reactions

“…This geometry is a result of electron donation from the bisphosphine ligand into the 3d(x 2 -y 2 ) orbital, which is also engaged in d to p* backbonding. 32,33 The measured K eq for the displacement of COD from 1 using benzaldehyde (>20), benzophenone (0.65), and acetophenone (0.02) are sufficiently large that under catalytic conditionsi.e. in the presence of a large excess (ca.…”

Large vs. Small NHC Ligands in Nickel(0) Complexes: The Coordination of Olefins, Ketones and Aldehydes at [Ni(NHC)2]

“…Selected bond lengths [Å] and angles [°] of 3: Ni1-C1 1.900(4), Ni1-C2 1.909(4), Ni1-C3 1.971(4), Ni1-C4 1.973 3, C3-C4 1.405 5, C1-N1 1.382 5, C1-N2 1.383(4), C2-N3 1.376(4), C2-N4 1.377(4), C1-Ni1-C2 131.01 15, C1-Ni1-C3 92.09(16), C2-Ni1-C4 96.22 13, C3-Ni1-C4 41.74 15, N1-C1-N2 101.79 (27), N3-C2-N4 101.65 (27) plane (C1-Ni1-C2) -plane (C3-Ni1-C4) 13.78(24). Selected bond lengths [Å] and angles [°] of 4: Ni1-C1 1.948(2), Ni1-C2 1.923(2), Ni1-C3 1.961(2), Ni1-C4 2.009(2), C3-C4 1.426 3, C1-N1 1.381 3, C1-N2 1.371 3, C2-N3 1.380(2), C2-N4 1.382 3, C4-C5 1.443 3, C5-O1 1.224 3, C5-O2 1.368 3, O2-C6 1.430 3, C1-Ni1-C2 125.58(9), C1-Ni1-C4 97.62(9), C2-Ni1-C3 94.67(9), C3-Ni1-C4 42.08(9), N1-C1-N2 101.88 17, N3-C2-N4 101.46 (17) plane (C1-Ni1-C2) -plane (C3-Ni1-C4) 3.37 (12). Table 1.…”

“…Love and Kennepohl et al published recently a study on the stabilization of square planar d 10 nickel π-complexes. [17] 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) was probed using a combination of 31 P NMR, Ni K-edge XAS, Ni K b XES, and DFT calculations. They have demonstrated that these complexes are best described as square planar d 10 complexes with π-backbonding acting as the dominant contributor to bonding to the π-acidic ligand.…”

“…22(8), plane (N3-C2-N4) -plane (Ni1-O1-C3) 72.55(10), plane (N1-C1-N2) -plane (N3-C2-N4) 66.18 (11). Selected bond lengths [Å] and angles [°] of 12: Ni1-C1 1.957(4), Ni1-C2 1.902(4), Ni1-O1 1.913(2), Ni1-C3 1.923 3, O1-C3 1.333(4), C1-N1 1.376(4), C1-N2 1.377(4), C2-N3 1.382(4), C2-N4 1.375(4), C1-Ni1-C2 130.89 14, C1-Ni1-O1 92.63 (12), O1-Ni1-C3 40.67 (12), C2-Ni1-C3 95.84 14, N1-C1-N2 101.8(3), N3-C2-N4 101.9(3), plane (C1-Ni1-C2) -plane (Ni1-O1-C3) 3.05(18), plane (N1-C1-N2) -plane (Ni1-O1-C3) 69.90 (17), plane (N3-C2-N4) -plane (Ni1-O1-C3) 63.45 20, plane (N1-C1-N2) -plane (N3-C2-N4) 50.77 (22). of the complexes 6, 11, 12 and the analogous phosphine complex [Ni(PCy 3 ) 2 (η 2 -O=CHPh)] D are summarized in Table 4.…”

“…European Journal of Inorganic Chemistry involvement of radical side reactions, since for such electronpoor π-systems metal-centered backbonding increases and it has been shown that nickel(I) character becomes significantly more important. [17] The 1 H NMR spectrum of 17 shows one set of signals for the carbene ligands with four singlet resonances at 2.00, 2.35, 6.02 and 6.84 ppm. The resonances of the aromatic protons of the carboxylate ligand can be found as two multiplets at 7.26 ppm and 7.91 ppm.…”