我們利用密度泛函理論(density functional theory)搭配NBO(Natrual Bond Orbital)分析法對三個系列的錯合物M(CO)6 {M=Cr, Mo, W}、M(CO)5X- {X-=F-, Cl-, Br-}及W(CO)5PR3 {PR3= PphMe2, Pph2Me, Pph3}之CO 2π電子雲密度進行探討。
　　NBO顯示在過渡金屬羰基錯合物M(CO)6q中，除了3d< 4d< 5d之傳統dπ(M)→2π(CO)的交互作用(donor-acceptor interaction)外，亦發現有兩種強度頗為可觀的交互作用。首先是相鄰羰基間5σ(CO)→2π(CO)之交互作用，其次是相鄰羰基接受金屬碳鍵σ(M-CO)→2π(CO)之交互作用，兩種作用大小均為3d> 5d~ 4d，而且在第六族過渡金屬羰基錯合物尤其顯著。因此第六族錯合物之CO 2π實驗值之大小趨勢(W>Cr>Mo)，可由這兩種交互作用的大小趨勢搭配dπ(M)→2π(CO)之交互作用來解釋。此外，本研究也詳細討論了上述兩種交互作用的情形。
　　在M(CO)5X-中，NBO顯示dπ(M)→Ry*(X)之電荷轉移量與E(2)大小趨勢均為Br->Cl->F-，此趨勢與鹵素取代基donate電子至金屬的能力相反，因此抵銷了鹵素配位基在錯合物M(CO)5X-中所預期的效應。M(CO)5X-之特殊的電子方向性亦可由NBO E(2)值來解釋，研究發現dπ-pπ作用力極有利於對位的CO。
最後，我們也透過NBO來探討W(CO)5PR3穿越效應，我們發現這些錯合物中配位基PR3上苯環與CO 2π的交互作用相當小，相對於其他的donor而言，電子由配位基PR3的苯環donate至CO 2π軌域的電子數目十分有限。

　　The NBO results indicate that besides the conventional dπ(M)→2π(CO) donor-acceptor interaction, in the uniform order 3d< 4d < 5d for each group under studies, there are two other significant D-A interaction. Firstly, the 5σ(CO)→2π(CO) indication which reveals the trend 3d>5d~4d. The secondtype interaction, σ(M-C) →2π(CO), which also exhibits the same trend. This trend is partically evident for the 6B group. Therefore, the experimental results observed for the 6B group can be understood as the latter two interactions tend to compatible with the dπ(M)→2π(CO) interaction. The detailed discussion of σ(M-C) →2π(CO) is also presented.
　　Analyzed of charge transfer and values of E(2) for the dπ(M)→Ryd(X) interaction in series 2 complexes reveals that the trend of this interaction, Br-> Cl-> F-. This reverse order of the halides’ donor capability, therefore, will counterbalance the expected ligand nature of the three ligands.
　　The unusual directionality found for M(CO)5X- can also be explained by the NBO results, In which the dπ-pπ interaction is orientational favored for the trans-CO. It is therefore not surprised that one would find the ligand effect is much more pronounce for the trans-CO than the radial ones.
Finally, NBO result indicates that the effect through space interaction between CO and benzene on phoshine in W(CO)5PR3 is limited. And, the result and NMR experiments have the same tendency.

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