Natural bond orbital parent

Ab initio calculations (MP2/6-31G ) of the parent compound of 8 revealed that the most stable arrangement of the dimer adopts Dih symmetry (Fig. 5). Interestingly, the four Li ions and the two phosphorus centers constitute an octahedral skeleton with relatively short Li-Li and Li-P distances of 2.645 and 2.458 A, respectively. Charge analysis (22) undoubtedly supports the electrostatic bonding model for this system because of the high net charges of the natural atomic orbitals (NBO) at Li (+0.768) and P (-1.583), while NBO-Lewis resonance structures support stabilization through delocalization (Fig. 5). 

To summarize, it seems that the bonding of dienes is complex and cannot be adequately represented by any one of the structures A, B or C. For those dienes which have substituents on the 1 4 carbons which are bent well away from the metal, representation B seems the most suitable. However, the relative importance in the raetal diene bonding of the y>i-4 diene orbitals will vary in different complexes. Therefore in this book we will use the representation A for all diene-metal bonds since it clearly indicates the diene nature of the parent olefin. 

The molecular and electronic structures of cyclic disulfide cation radicals of 1,2-dithietane 6 and 1,2-dithiete 7, and radical cations of 1,2-dithiolane 2 (2a-c represent stable conformations determined in terms of the symmetry restriction of Cs, Cz, and Czv), with emphasis on the nature of a two-center three-electron (Zc-ie) sulfur-sulfur bond have been examined by ab initio molecular orbital (MO) calculations <1997JMT(418)171>. Unrestricted Hartree-Fock (UHF)/ MIDI-4(d) computations showed that this bond in organodisulfide radical cation 2 is shorter in comparison to 1,2-dithiolane 2 and possesses partial Jt-bond character (structure A), as previously implied by electron spin resonance (ESR) spectroscopy <1982JA2318>, which correlates best with the form as the most favorable conformation of the cation radical 2. Contrary to the repulsive S-S interaction in the parent 1,2-dithiolane arising from the lone pairs of electrons, the hemi-7t-bond formed by one-electron oxidation should stabilize the five-membered ring of 2, or, for example, a similar cation radical of LA 3 which is involved in diverse biochemical reactions. 

The dominance of ionic bonding and the -i-3 oxidation state in 4f complexes is attributed to the core-like nature of the valence 4f orbitals [1]. From the parent Ln electronic configuration, [Xe]6s 5d 4F (n= 1-14), the 6s and 5d electrons are easily ionized to give [Xe]4T Ln configurations. For several of the Ln series, notably Yb, Sm, and Eu, the molecular chemistry of the +2 oxidation state is rich and well... 

The detailed symmetry-analysis by Trindle and the author [17] of the parent reaction is summarized concisely in Fig. 10.3. Two 0 atoms, two C atoms and two H atoms provide twenty-two valence electrons altogether. Ten of these six in the CC and CH bonds of ethylene and four oxygen lone-pair electrons - need not be considered explicitly, because the five orbitals housing them retain their symmetry properties across the diagram. Fig. 10.3 makes do with six MOs for the closed shell dioxetene intermediate these are supplemented by one more in the reactants and product, where open shell states have to be taken into account. The ordering of the orbitals in the three species is intuitive, but - apart from the upper two, that determine the nature of the lowest excited states - it is immaterial. 

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