Ligands Having Extended Pi Systems

We will first describe linear and then cyclic tt systems, after which we will consider how molecules containing such systems can bond to metals. 

The simplest case of an organic molecule having a linear tt system is ethylene, which has a single rr bond resulting from the interactions of two 2p orbitals on its carbon atoms. Interactions of these p orbitals result in one bonding and one antibonding tt orbital, as shown  

The antibonding interaction has a nodal plane perpendicular to the intemuclear axis, but the bonding interaction has no such nodal plane. 

Next is the three-atom tt system, the rr-allyl radical, C3H5. In this case, there are three 2p orbitals to be considered, one from each of the carbon atoms participating in the TT system. The possible interactions are as follows  

The number of nodes perpendicular to the carbon chain increases in going from lower-energy to higher-energy orbitals for example, in the rr-allyl systan, the number of nodes increases from zero to one to two from the lowest to the highest energy orbital. This is a trend that will also appear in the following examples. 

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