A first example the octahedral complex

Consider an octahedral complex with one Cl ligand (or any other halogen) which is a double-face tt-donor ( 3.1.2) (3-8) the five other ligands only have cr interactions with the metal. 

In order to construct the d block of this complex, we start from the orbitals that were obtained by only considering a interactions, then we perturb them by introducing the n interactions. The d block derived from these a interactions will be assumed identical to that already established for a fully octahedral complex (MLg), that is, split into two degenerate groups (t2 and e, Chapter 2, 2.1.2.4). This is of course a simplification, since in an [ML5CI] complex, the a interaction created by one of the L ligands is different from that involving the Cl.  

By examining the symmetry properties of the d orbitals of the regular octahedron as well as those of the tt donor, we can predict which ones will be perturbed by the tt interactions. In the case of the [ML5CI] complex (3-8), it is convenient to use the symmetry planes xz (Pi) and yz (P2) (3-9). 

The xy orbital is antisymmetric with respect to the two planes (AA), xz is symmetric with respect to Pi but antisymmetric with respect to 2 (SA), while yz is antisymmetric with respect to Pi but symmetric with respect to P2 (AS). The antibonding orbitals, x —y and z, are symmetric with respect to both planes (SS). On the chlorine atom, the lone pairs (px and Py) have SA and AS symmetries, respectively (3-10). 

The influence of the electronic configuration on the 7t-donor character of the Cl ligand 

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A first example the octahedral complex [ML5CO] 3.2.4.1. Interaction diagram... 

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