Extended Huckel Schemes

These arguments go hand in hand with Extended Huckel Theory (EHT), both being based on overlap (symmetry) considerations. In fact, an EHT calculation will provide almost exactly the same results as a skilful use of the qualitative MO building scheme we have provided in this section. 

Fig. 10. Schemes of MO s for technetium compounds according to the results of extended Huckel calculations [58]...

In both the extended Hartree-Fock calculation and the valence bond calculation effects of spin correlation are included, but not in the simple Huckel scheme. The x-energy levels for the allyl radical arc shown schematically below. 

The methylidyne cubane [Cp Ti(/x-CH)]4 (Scheme 202) is obtained as a dark brown crystalline solid by thermolysis of Cp TiMe in toluene with methane elimination. This transformation was monitored by NMR and no intermediates are observed. The signals assignable to the methylidyne groups appear as singlets at 6 17.75 in the NMR and at 6 490.8 in the 13C NMR spectra. In order to analyze the interaction between the /i3-ligand and the titanium centers, extended Huckel molecular orbital calculations have been carried out. In contrast, the thermolysis of the trinuclear oxo alkyls [Cp Ti(/i-0)(CH2R)]3 (R = H, Me) affords the //3-alkylidyne derivatives [Cp Ti(//-0)]3-(/x3-CR).505-507... 

Historically the extended Hiickel model (EHT, Extended Huckel Theory, as it was originally called) is one of the most important schemes developed. Even in its simplest form, orbitals with energies approximating ionization potentials and of the proper nodal structure and symmetries are obtained. The famous Woodward-Hoffmann rules were founded on these simple calculations, and frontier molecular orbital arguments are easily based on EHT orbitals. 

Figure 2.9. Extended-Huckel II molecular orbital energy scheme.

Another mechanistic problem concerned the involvement of the bicyclic intermediates, 160 and 162 (Scheme 30). Compound 162 has been postulated for many years as the intermediate which is trapped by nucleophiles in the thermolysis or photolysis of aryl azides, leading to azepines (this reaction is described in detail in Section VIII,C). However, the present author pointed out that there was no compelhng evidence for 162, and that the trappable intermediate could equally well be the seven-membered ring 161. The question then arises is 163 161 a one-step reaction, or does it take place via 162 (see Scheme 30). CNDO/2 and extended HUckel calculation failed to answer this question unambiguously, but both methods showed that the nitrene N in 163 must move out of the plane of the ring en route to 161. The calculations by Shillady and Trindle favored 162 as a stable intermediate. There is no doubt of the existence of carbocyclic analogs of 162, which can be trapped in solution [e.g., Eq. (45)]. ... 

Other low-temperature photolysis experiments have investigated ejection of CO from [(arene) MCCOja], followed by oxidative addition of H—SiRa/ Kinetic studies revealed a iC -fold range in reactivity for [(arene)M] = [(C6H6)Cr] > [CpMn] > [(C4H4)Fe] > [CpRe]. This reaction was also modeled by the extended Huckel method/ Several new systems have been found to activate alkanes. In an extension of previous work, [Cp Ir(H)(i7 -allyl)] has been found to generate cr-alkyl, a-propyl compounds via the mechanism in Scheme When R has... 

The abovementioned behaviour has been rationalized,at least partly, by simplified qualitative tt-MO schemes [23] and also by means of extended Huckel calculations [24] which indicate that, although the HOMO of the isocyanide complex is mainly localized at the chloride ligand and at the metal atom, the nitrogen atom carries a considerable negative net atomic charge (in contrast with the metal and the ligating isocyanide carbon atom). Therefore, the 3-protonation appears to be charge-controlled rather than frontier-orbital controlled [24]. 

Hamiltonian is not known and, as for the nonrelativistic case, further approximations have to be introduced in the wavefunction, it is tempting to derive approximate computational schemes which are still sufficiently accurate but more efficient. Here we will only summarize those approximate methods that have been used frequently to obtain information about the electronic structure of molecules with lanthanide atoms, i.e. relativistically corrected density-functional approach, pseudopotential method, intermediate neglect of differential overlap method, extended Huckel theory, and ligand field theory. 

Finally, the Ag-Ag distances in adjacent pairs of the photoluminescent ion [AgfCNlj]" decrease by as much as 0.58 A upon electronic excitation. Extended Huckel molecular orbital calculations indicate the formation of an exciplex, with a transition from a la la 2o 2a ground state with no formal bond (analogous to the orbital scheme shown in Eigure 11.4 for two Cu(l) ions), to a lo la 2o 2o ... 

We start with discussing the simplest case, the Huckel scheme. This model has been introduced into quantum chemistry a long time ago (Huckel 1930), and practically it still represents the only possibility to obtain (at least semi-quantitative) information about extended Tc-electron systems containing several dozens (or hundreds) of atoms (Kertesz Suijan 1981, Suijan Kuzmany 1986). 

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