Srebro Monika, Michalak Artur
Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland.
Inorg Chem. 2009 Jun 15;48(12):5361-9. doi: 10.1021/ic900336r.
The natural orbitals for chemical valence and the Ziegler-Rauk bond energy decomposition analysis were used to describe the donor/acceptor character of the N-heterocyclic carbenes (NHC)-metal bond in two groups of square-planar rhodium(I) complexes: (NHC)RhCl(cod) (1-X; cod = 1,5-cyclooctadiene) and (NHC)RhCl(CO)(2) (2-X), with a group X = H, Cl, NO(2), or CN located on the NHC ligand. The results show that the NHC-metal bond consists of the components originating from donation (sigma symmetry) and back-donation (two contributions of the pi symmetry, out-of-plane and in-plane, accompanied by one sigma-back-bonding component). The charge-flow measures from NOCV indicate that the total back-bonding contribution is of comparable importance to donation. The out-of-plane pi component contributes to ca. 50% of the total back-bonding charge-flow. The energy measures from the Ziegler-Rauk analysis show that the total back-bonding energy corresponds to ca. 40% of the orbital interaction energy. The ligand trans to NHC (CO or cod) strongly affects the back-bonding component; for the complexes 1-X, the back-donation is substantially enhanced compared to 2-X. The back-bonding component increases with an increase in the pi-withdrawing ability of X for both, 1-X and 2-X. However, this effect is relatively small. Back-bonding components of the two bonds involving the metal are strongly coupled; an increase in NHC-Rh leads to a decrease in Rh-olefin/CO(trans). The changes in the back-bonding are too small to be followed by the trends in bond energies, which are finally determined by the electrostatic and Pauli repulsion energy.
利用化学价自然轨道和齐格勒-劳克键能分解分析，描述了两组平面正方形铑（I）配合物中氮杂环卡宾（NHC）-金属键的给体/受体特征：（NHC）RhCl（cod）（1-X；cod = 1,5-环辛二烯）和（NHC）RhCl（CO）₂（2-X），其中NHC配体上的基团X = H、Cl、NO₂或CN。结果表明，NHC-金属键由源自给予作用（σ对称性）和反馈作用（π对称性的两种贡献，面外和面内，伴有一个σ-反馈键成分）的成分组成。来自NOCV的电荷流测量表明，总的反馈作用贡献与给予作用相当重要。面外π成分对总反馈电荷流的贡献约为50%。齐格勒-劳克分析的能量测量表明，总的反馈键能约相当于轨道相互作用能的40%。与NHC反位的配体（CO或cod）强烈影响反馈键成分；对于配合物1-X，与2-X相比，反馈作用显著增强。对于1-X和2-X，随着X的吸π电子能力增加，反馈键成分增加。然而，这种影响相对较小。涉及金属的两个键的反馈键成分强烈耦合；NHC-Rh的增加导致Rh-烯烃/CO（反位）的减少。反馈键的变化太小，无法通过键能趋势来跟踪，键能最终由静电和泡利排斥能决定。
