本研究首度以NBO(Natural Bond Orbital)方法對一氧化碳金屬錯合物進行Pi鍵結電子的理論研究。結果顯示,在同一族間呈現兩種不一樣的回饋鍵結電子密度傾向: (1)6B、7B族錯合物:5d>3d>4d,此結果符合多數近期的研究,(2)4B、5B、8B族錯合物: 5d>4d>3d,其中4B、5B族則與文獻上IR研究結果一致。由E(2)值分析發現,回饋電子來源可以歸納為兩大類:第一類來自於中心金屬原子dπ→ 2π(CO),其軌域間的穩定能量E(2)值呈現: 5d>4d>3d,分析其原因是來自作用軌域間的能量差(Ej-Ei)值: 5d<4d<3d,以及軌域間的作用能量(Fij)呈現: 5d>4d>3d，此結果如一般所預期的傾向。第二類是來自於錯合物順式位置上的碳原子pπ→ 2π(CO),其E(2)值傾向則與第一類不同,而呈現: 3d>5d>4d,其原因是由於作用軌域間的能量差呈現: 3d<4d<5d,而軌域間的作用能量則呈現: 3d>5d>4d。這個結果與dπ→ 2π(CO)的傾向有所衝突,因而造成大多數6B、7B族金屬六羰基錯合物2π(CO)軌域回饋鍵結電子密度呈現: 3d≈5d>4d傾向。
　　本研究也對於一些6B族M(CO)5L配位基的本質作比較,結果可以歸納出這些配位基πeffect強度依序為: CO >SiO≈ H2>CN¯。尤其本研究發現CN¯是一強σdonor,幾乎近於π中性,此點與文獻Mossbauer光譜的結果非常接近。但對H2配位基而言,他們的實驗方法似乎高估了H2的πacceptor性質。至於SiO配位基,我們發現應是一個可以穩定錯合物的配位基,因其π效應僅略小於CO。這些配位基的[2π]值,也符合本研究對於6B及7B族呈不規律之傾向結果。

　　The natural populations in the carbonyls’ antibonding pi (2p) orbitals have been calculated by Natural Bond Orbital (NBO) method. There are two types of tendency found for a given family. For groups 6B and 7B, the order of backbonding extend acquired is 5d > 3d > 4d ( Class A). For the rest series, 4B, 5B and 8B, however, the tendency of backbonding obtained is as expected 5d > 4d > 3d (class B). The 2p population arising from two orbital interactions: dπ(M)→ 2p and pπ(C)→ 2p according to the NBO method. The dp(M)à2p reveals the 5d>4d>3d order whereas the pπ(C)→ 2p indicates a 3d>5d>4d order for class A M(CO)6. The overall results found in groups 6B and 7B, therefore, can be accounted by the net effects of the above-mentioned mechanisms. In class B, on the other hand, the dπ(M)→ 2p dominates and one finds the expected order based purely on characteristics of nd-orbitals.
　　The same studies have been extended on group 6B M(CO)5L in order to examine the nature of substitute L, where L= CN¯, SiO and H2 in the current study. Through natural population analysis on either the remained carbonyl’s 2p or the ligand’s Rydberg orbitals, we have found the strengths for p-accepting capability are in the order CO > SiO≈ H2 > CN¯. It is noteworthy that our results reveal that the cyano ligand is nearly p-neutral or is better treated as a s-donor ligand. This conclusion provides support in support of one current report regarding to cyano from Mossbauer spectroscopy. Whereas the H2 ligand is a rather weak p-acceptor. Moreover, NPA performed on this series also indicates the specific order for group 6B transition metals,5d > 3d > 4d.

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