| Summary: | 碩士 === 國立成功大學 === 化學系專班 === 92 ===
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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