Donor鈥揂cceptor Properties of Bidentate Phosphines. DFT Study of Nickel Carbonyls and Molecular Dihydrogen Complexes

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• 作者：Charity Flener Lovitt ; Gernot Frenking ; Gregory S. Girolami
• 刊名：Organometallics
• 出版年：2012
• 出版时间：June 11, 2012
• 年：2012
• 卷：31
• 期：11
• 页码：4122-4132
• 全文大小：485K
• 年卷期：v.31,no.11(June 11, 2012)
• ISSN：1520-6041

文摘

Density functional theory (DFT) is used to investigate the geometries and metal鈥搇igand bonding in nickel complexes of bidentate phosphines, NiX<sub>2sub>(R<sub>2sub>P(CH<sub>2sub>)<sub>nsub>PR<sub>2sub>), where X = H, CO, n = 1鈥?, and R = H, Me, CF<sub>3sub>, Et, i-Pr, t-Bu, Ph, OMe, F. The net donor鈥揳cceptor properties of the phosphine ligands can be deduced from the computed frequency of the symmetric CO stretch of the Ni(CO)<sub>2sub>(R<sub>2sub>P(CH<sub>2sub>)<sub>nsub>PR<sub>2sub>) carbonyl complexes. This frequency (in cm<sup>鈥?sup>) can be estimated from the empirical expression 谓(CO) = 1988 + 鈭懴?sub>Bsub> 鈥?4n, where the sum is over the four substituents on the bidentate phosphine, 蠂<sub>Bsub> is a substituent-dependent parameter, and n is the number of carbon atoms in the backbone (1 鈮?n 鈮?3). The deduced values of 蠂<sub>Bsub> (in units of cm<sup>鈥?sup>)鈥攖-Bu (0.0), i-Pr (0.8), Et (3.0), Me (4.0), Ph (4.3), H (6.3), OMe (10.8), CF<sub>3sub> (17.8), and F (18.3)鈥攁re generally similar to Tolman鈥檚 electronic parameter 蠂 derived from nickel complexes of unidentate phosphines. For the NiH<sub>2sub>(R<sub>2sub>P(CH<sub>2sub>)<sub>nsub>PR<sub>2sub>) 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<sup>鈥?sup> 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鈥揹ihydrogen interaction involves a three-center鈥搕wo-electron bond. The Kohn鈥揝ham molecular orbital diagram and energy decomposition analysis of these complexes show that metal to H<sub>2sub> 蟺 back-donation is the dominant orbital component for phosphines with electron-donating substituents, whereas H<sub>2sub> to metal 蟽 donation is dominant for phosphines with electron-withdrawing substituents. The EDA results clearly indicate that long H鈥揌 distances are seen when the metal to H<sub>2sub> 蟺 back-donation dominates over H<sub>2sub> to M 蟽 donation.