Molecular orbital octahedral case

Although we will not write the complete wave functions as we did for the case of an octahedral complex, the molecular orbitals give rise to the energy level diagram shown in Figure 17.20. 

We apply molecular orbital theory to octahedral coordination complexes just as we did for the simple metal carbonyl in Section 8.2. We begin by constructing the cr MOs from the valence d, s, and p orbitals of the central metal atom and the six ligand orbitals that point along the metal-ligand bond directions in an octahedral complex. In the case of the Cr complexes we use as examples, the relevant... 

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 molecular orbital theory of complexes (6), developed out of crystal field theory (7) by the inclusion of both a- and sr-covalent interactions, has been successfully applied 8—13) to the octahedral (14. 15) carbonyls of the 4 metals (V i, Cr , Mn+ etc.). The qualitative discussion of this octahedral case is particularly simple because of a clean-cut separation (within the single-configuration approximation) of a- and ji-systems. 

The naive application of the picture of Fig. 1 to Ni(C0)4, for example, is totally misleading. It is assumed that the metal can present each CO with one empty pure metal-carbon bonds. This is legitimate for the octahedral case, but not for tetrahedra. The defect is readily removed by a qualitative molecular orbital approach, illustrated in Fig. 3. 

However, before looking at the results of these calculations, we believe that a few important points can already be made based on the octahedral MO scheme in Figure 1 and the considerations of the previous section. Thus, as a first case, suppose that we are dealing with the extreme situation of a truly ionic transition metal system. In such a case all molecular orbitals are either entirely ligand or entirely metal based. The ligand valence orbitals are fully occupied and at considerably lower energy than the metal 3d orbitals, with which they do not interact. Therefore, one may in this case expect important correlation effects to occur only within the 3d valence shell. This means that a reference CASSCF calculation on such a system should include only the metal 3d, and possibly a second 3d shell (see Sec. 2). 

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