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

Density functional theory (DFT) is used to investigate the geometries and metal-ligand bonding in nickel complexes of bidentate phosphines, NiX2(R2P(CH2)(n)PR2), 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)(n)PR2) carbonyl complexes. This frequency (in cm(-1)) can be estimated from the empirical expression nu(CO) = 1988 + Sigma chi(B) - 4n, where the sum is over the four substituents on the bidentate phosphine, chi(B) is a substituent-dependent parameter, and n is the number of carbon atoms in the backbone (1 <= n <= 3). The deduced values of chi(B) (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 chi derived from nickel complexes of unidentate phosphines. For the NiH2(R2P(CH2)(n)PR2) hydride complexes, the global minimum is a nonclassical dihydrogen structure, irrespective of the nature of the phosphine. For bidentate phosphines that are strongly donating, a classical cis-dihydride structure lies higher in energy (in some cases, by only 0.4 kcal mol(-1) above the global minimum). For phosphines that are less electron donating, the dihydride structure is no longer a local minimum but instead is an inflection point on the potential energy surface. Atoms in molecules (AIM) and natural bond order (NBO) analyses confirm that the nickel-dihydrogen interaction involves a three-center-two-electron bond. The Kohn-Sham molecular orbital diagram and energy decomposition analysis of these complexes show that metal to H-2 pi back-donation is the dominant orbital component for phosphines with electron-donating substituents, whereas H-2 to metal sigma donation is dominant for phosphines with electron-withdrawing substituents. The EDA results clearly indicate that long H-H distances are seen when the metal to H-2 pi back-donation dominates over H-2 to M sigma donation.