Ligand antibonding

Ligand-centred (LC) transitions between bonding and antibonding ligand-centred MOs. These transitions are expected for aromatic ligands with extended n- and it -orbitals. 

Metal-to-ligand charge transfer (MLCT) transitions between the nonbonding metal-centred MOs and antibonding ligand-centred MOs. Such transitions are found where a metal is easily oxidised and the ligand is easily reduced. 

Transitions from -bonding to 7r-antibonding ligand orbitals, called ligand-centred (LC) or internal ligand (IL)... 

The antibonding ligand 7i -orbital is too high in energy to take part in bonding. 

The consequences of this backbonding are experimentally observable. X-ray crystal structures show that the C-O and C-C bonds of CO and C2H4 respectively are longer when these molecules are bonded to a transition metal than when they are free, which means a decrease of the bond order upon coordination. This bond lengthening is due to the population of the antibonding ligand orbital in the complex. Also, the infrared frequency of the carbonyl is lower for the CO ligand in the metal-carbonyl complex than for free CO, as a consequence of the decrease of the C-O bond order upon coordination. 

Jt Acceptor backbonding from the filled d metal orbital to antibonding ligand orbital. 

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