The Molecular Orbital Model of Bonding

In the MO bonding model, atomic orbitals which are of the same symmetry, similar energy and which have significant overlap are combined to produce an equal number of MOs. These are no longer localized at 

Hybridization in an octahedral complex in the valence bond model of coordination ompound the formation of bonds to an octahedral transition metal ion would involve six d sp h /hrid orbitals. In the approach used here the diff( ring syininetry properties of the s, thii p, p and p.. and the d, and d orbitals used in bonding are explicitly taken into account. However, the 3 all of the p and two ol the d metal valence shell orbitals are otill used in o-bond tormation. 

Although the MO model developed so far contains the features found in the CFT model, it does not yet account for the finding that anionic F is a weak-field ligand but neutral CO is strong field. To do this it is necessary to include in the model other orbitals associated with the ligand donor atoms, particularly those which can participate in n bonding. 

In the case of a ligand like CO the Ti-type p orbitals on carbon are involved in k bonding to oxygen. However, there is a set of 12 unoccu- 

A further point which emerges from this consideration of k bonding is that, in the presence of n donor ligands such as F, the metal electrons move to higher energy in the t2g orbital and so are easier to ionize. The 

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Whether a complex is high- or low-spin depends upon the energy separation of the t2g and eg levels. Nationally, in a fj-bonded octahedral complex, the 12 electrons supplied by the ligands are considered to occupy the aig, and eg orbitals. Occupancy of the and eg levels corresponds to the number of valence electrons of the metal ion, just as in crystal field theory. The molecular orbital model of bonding in octahedral complexes gives much the same results as crystal field theory. It is when we move to complexes with M—L TT-bonding that distinctions between the models emerge. 

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