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PC4. Photolysis

We have seen that the absorption of photons (especially in the ultraviolet-visible spectrum) is connected to the excitation of electrons.  After excitation, a number of different relaxation pathways lead back to the ground state.  Sometimes, absorption of a photon leads to a vastly different outcome.  Instead of just relaxing again, the molecules may undergo bond-breaking reactions, instead.

An example of this phenomenon is observed in the complex ion [Co(NH3)]63+.  Addition of UV light to this complex results in loss of ammonia.  In the absence of UV light, however, the complex ion is quite stable.

In many cases, loss of a ligand is followed by replacement by a new one.  For example, if an aqueous solution of  [Co(NH3)]63+ is photolysed, an ammonia ligand is easily replaced by water.

Problem PC4.1.

Draw a d orbital splitting diagram for  [Co(NH3)]63+.  Explain why this complex is normally inert toward substitution.

Problem PC4.2.

Use the d orbital splitting diagram for  [Co(NH3)]63+  to explain why this complex is undergoes substitution upon irradiation with UV light.


Photolysis is the term used to describe the use of light to initiate bon-breaking events.  Photolysis frequently involves the use of high-intensity ultraviolet lamps.  The high intensity light is needed in order to provide enough photons to get higher conversion of reactant into a desired product.