A comparison of the MO and VB bonding models

As we have already mentioned, it is essential to understand that, in MO theory, the bonding in a molecule is described by combining the characters of all the occupied MOs with bonding character. Take CH4 as an example. The orbital 

Powell (2004) Journal of Chemical Education, vol. 81, p. 1734- Improvements in the reliability of X-ray photoelectron spectroscopy for surface analysis . 

Reinert and S. Hufner (2005) New Journal of Physics, vol. 7, p. 97 - Photoemission spectroscopy - from early days to recent applications . 

Yang (2004) in Comprehensive Coordination Chemistry II, eds J.A. McCleverty and T.J. Meyer, Elsevier, Oxford, vol. 2, p. 182 - An overview of photoelectron spectroscopy, including the use of synchrotron radiation and laser-based techniques. 

As we have already mentioned, it is essential to understand that, in MO theory, the bonding in a molecule is described by combining the characters of all the occupied MOs with bonding character. Take CH4 as an example. The U orbital (Fig. 5.21) is spherically symmetric and provides equal bonding character in all four C—H interactions. The 2 orbitals must be considered as a set and not as individual orbitals. Taken together, this set of orbitals provides a picture of four equivalent C—H bonding interactions and, therefore, the overall picture is one of C—H bond equivalence. 

6 Molecular orbital theory bonding anaiyses soon become compiicated 

Ligand group orbitals involving the F 2s orbitals in BF3 and having and e symmetries can be derived in the same way as those for the H3 fragment in BH3. These are shown as LGO(l)-LGO(3) in Fig. 5.22. The p orbitals on 

This row of characters is also produced by summing the rows of characters for the 12 and E representations in the Djh character table (Table 5.1), and therefore the LGOs are of uf2 and e symmetries. By considering the effects of every operation on one of the F 2p orbitals in the F3 fragment, we can (as before) arrive at an equation for the unnormalized wavefunction of each LGO. Let the three F 2p orbitals be V l. V 2 d V s- We now generate 

Multiplying each character in this row by the corresponding character in the row for the A2 representation in the Z 3 character table (Table 5.1) gives the uimormalized form of the wavefunction for the a2 LGO (eq. 5.22). Simplification and normalization gives eq. 5.23. The 2 LGO can thus be described as an in-phase combination of Ip orbitals and is shown schematically in Fig. 5.22 as LGO(5). 

When we considered how valence bond theory can be used to describe the bonding in BH3, CH4 and NH3, we used appropriate hybridization schemes such that bonds known to be structurally equivalent would be equivalent in the bonding scheme. One hybrid orbital contributed to each localized X—H (X = B, C or N) bond. On the other hand, the results of MO theory indicate that the bonding character is delocalized. Moreover, in each of BH3, NH3 and CH4, there are two different types of bonding MO a unique MO involving the 2s atomic orbital of the central atom, and a degenerate set of two (in BH3 and NH3) or three (in CH4) MOs involving the 2p atomic orbitals of the central 

Excess energy of electron — E — (binding energy of electron) 

Since the excess energy can be measured and E is known, the binding energy can be determined. Koopmans theorem 

DeKock and H.B. Gray (1980) Chemical Structure and Bonding, Benjamin/Cummings, Menlo Park - This includes a more detailed discussion of the application of PES. 

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