Donor–Acceptor Properties of Bidentate Phosphines. DFT Study of Nickel Carbonyls and Molecular Dihydrogen Complexes

Abstract

Density functional theory (DFT) is used to investigate the geometries and metal–ligand bonding in nickel complexes of bidentate phosphines, NiX2(R2P(CH2)nPR2), where X = H, CO, n = 1–3, and R = H, Me, CF3, Et, i-Pr, t-Bu, Ph, OMe, F. The net donor–acceptor properties of the phosphine ligands can be deduced from the computed frequency of the symmetric CO stretch of the Ni(CO)2(R2P(CH2)nPR2) carbonyl complexes. This frequency (in cm–1) can be estimated from the empirical expression ν(CO) = 1988 + ∑χB – 4n, where the sum is over the four substituents on the bidentate phosphine, χB is a substituent-dependent parameter, and n is the number of carbon atoms in the backbone (1 ≤ n ≤ 3). The deduced values of χB (in units of cm–1)—t-Bu (0.0), i-Pr (0.8), Et (3.0), Me (4.0), Ph (4.3), H (6.3), OMe (10.8), CF3 (17.8), and F (18.3)—are generally similar to Tolman’s electronic parameter χ derived from nickel complexes of unidentate phosphines. For the NiH2(R2P(CH2)nPR2) hydride complexes, the global minimum is a noncl...