A single ligand orbital cr interactions

The case where it is clearest that only one orbital need be considered involves the ligand H, since it possesses only one valence orbital, This orbital, which contains one electron (an X-type ligand), can be used to form a cr —h bond by combination with a metal orbital such as the orbital (1-32).  

For other ligands the situation is more complicated, since it is necessary to take several orbitals into account to obtain a satisfactory description of the bond with the metal. As a first example, we shall consider bent AH2 molecules. 

As in the previous example, the bonding and antibonding orbitals can, in a first approximation, be neglected for the description of the metal-ligand interactions. However, it is necessary to take both nonbonding orbitals into account, as they are close in energy and both can lead to interactions with the metal centre. The 2ai orbital plays the 

The Ibi orbital (p ) has the correct symmetry to interact with the xz orbital the overlaps above and below the yz plane have the same sign, so that the total overlap (the sum of the partial overlaps) is non-zero (1-36). This new metal-ligand interaction is said to be a r interaction, since the orbitals concerned share a common nodal plane (yz). It leads to the formation of two MO, one bonding, jtm-l. represented schematically in 1-36, and one antibonding, This interaction is particularly important 

With the exception of the ligand FI, for which there is only a single valence orbital Ish to consider (a a interaction), one must treat aU the valence s and p orbitals on a monoatomic ligand A. Now the s orbital is usually much lower in energy than the d orbitals on the metal, especially 

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