锇杂苯的电子结构及芳香性

使用密度泛函理论及片段轨道相互作用分析方法,研究了典型的锇杂苯的电子结构和芳香性.结果表明,锇碳六员环具有较好的环平面性及键的离域性,占据的锇dxz轨道与碳环的3π空轨道之间的反馈π键相互作用,使得环上离域π电子数满足Hückel的4n+2规则,计算的环外质子化学位移、同键反应芳香性稳定化能、绝对硬度和磁化率增量数据均表明锇杂苯具有芳香性.

Electronic Structure and Aromaticity of Osmabenzene

Electronic structure and aromaticity of the typical osmabenzene complex have been investigatedwith the aid of qualitative fragment orbital interaction analysis and density functional theory calculations. The model osmabenzene has been fully optimized at the B3LYP level. And the results showthat the theoretical calculationswell reproduce the experimental geometry. The six-membered metallacycle in the osmabenzene keeps very good coplanarity and also the average of bond lengthes has been observed, which is the typical characteristic of aromaticity. The valence electron analysis shows that the six-membered metallacycle possesses six delocalizedπ-electrons due to the back-bonding interaction between the occupied metal dxz-orbital and the empty 3πorbital of the carbon unit, i.e. dxz(Os)-3π(C5H5-), and therefore obeys the Hückel 4n+2 rule. The bending away fromthe carbon unitof the P(phosphine)-Os-P(phosphine) can also enhance the dxz(Os)-3π(C5H5-) back-bonding and benefit the aromaticity of the metallacycle, which is consistentwith the calculated smaller P-Os-P angle. The NMR chemical shift, homodesmotic reaction aromatic stabilization energy (HASE), absolute hardness, susceptibility exaltation of the model osmabenzene as well as benzene and pyridine have been also investigated. The calculated chemical shifts of the ring protons in the metallacycle of the osmabenzene are obviously shifted downfield, which mainly stem from the cyclic current in the metallacycle. And an extreme case is that the twoα-Hs, which are nearly close to the osmium, have shifted much more. The change can be attributed primarily to the magnetic anisotropic influences of the adjacent large metal atom. The calculated HASE of the osmabenzene is negative, which demonstrates that the resonance energy exists in the studied system. The calculated absolute hardness of the osmabenzene is 4.43 eV.