Antibonding molecular orbitals octahedral complexes

One of the simplest cases of it bonding in octahedral complexes is found in [CoFJ . Its fT system will be similar to that in Fig. 11.20. The orbitals of the metal can interact with /ijt, LGOs constructed from the fluorine 2p orbitals to form rr-bonding and antibonding molecular orbitals. Since fluorine is more electronegative than cobalt. 

The 3dj2 and 3d 2 2 orbitals of an octahedral complex, along with the 4s and three 4p orbitals are assumed to overlap six ligand orbitals, forming six bonding molecular orbitals and six antibonding molecular orbitals. 

The molecular orbital model can also be applied to complexes of the d-block elements. In octahedral complexes the d-orbitals of the metal are not degenerate, as they are in the free metal, because of the interaction between the ligand and metal orbitals. The five d-orbitals are split into three t2g (nonbonding) and two e (antibonding) MOs that is ... 

A representative molecular orbital diagram for an octahedral d-block metal complex ML6 is shown in Figure 1.8. The MOs are classified as bonding (oL and ttL), nonbonding (jtM) and antibonding (o, nl and ). The ground-state electronic configuration of an octahedral complex... 

The sequence of energy levels obtained from a simple molecular orbital analysis of an octahedral complex is presented in Fig. 1-12. The central portion of this diagram, with the t2g and e levels, closely resembles that derived from the crystal field model, although some differences are now apparent. The t2g level is now seen to be non-bonding, whilst the antibonding nature of the e levels (with respect to the metal-ligand interaction) is stressed. If the calculations can be performed to a sufficiently high level that the numerical results can be believed, they provide a complete description of the molecule. Such a description does not possess the benefit of the simplicity of the valence bond model. 

A detailed interpretation of these results has not been given. However, nucleophilic substitution at silicon can be accepted as a frontier orbital controlled process between the HOMO of the nucleophile and the antibonding orbital as LUMO165. There are important differences in energy levels between octahedral and pentagonal complexes, and molecular orbital calculations might suggest a rationalization of these differences. 

Figure 1 Molecular orbital diagram for an octahedral complex with o--only Interactions. Population of the molecular orbitals by 18-electrons results In occupation of six bonding and three nonbonding orbitals, with no occupation of antibonding orbitals. As such, it provides a simple rationalization for the concept of the 18-electron rule a 20-electron complex with occupied antibonding orbitals would be expected to be unstable and dissociate a ligand to transform to an 18-electron complex, while a 16-electron complex would be able to bind an additional ligand and transform to an 18-electron complex. However, the nature of the molecular orbital diagram depends critically on the molecular structure (for example, see Figure 2) and so this explanation is necessarily over simplistic.

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