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.