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Study Notes: Absorption of UV-Vis Radiation by Transition Metal Complexes

The UV-Vis spectra of transition metal complexes arise as a result of electronic transitions just as they do in organic compounds. There can be more than one type of electronic transition, or excitation, taking place depending upon the nature of the chromophore(s) involved. The spectrum that you see is the combination of the different types of transitions as they occur within the compound.

For instance, where a complex has a ligand that is an organic compound containing saturated bonds, such as with pyridine, the ligand will be excited and absorb UV radiation in the same way as it would do on its own. The absorption due to the pyridine ligand therefore forms part of the spectrum for the particular complex.

We shall concentrate on the electronic transitions that principally give rise to absorption in the visible region and are therefore responsible for the colour of transition metal complexes. These are known as d-d transitions and relate to excitation of the metal ion itself.

Transition metals are often defined as forming one or more stable ions with incompletely filled d orbitals. It is believed that these are involved in generating colour.

When ligands bond to a transition metal ion to form a complex, electrons in the ligands and electrons in the five d orbitals of the metal ion repel each other. The net effect is that the energies of the d orbitals are raised, however they are split into two groups of differing energy.

Representation of the five d copper orbitals being split into 2 groups upon bonding with the 6 water ligands.

The arrangement relates to the d orbital electrons (9 in total ) in a Cu2+ ion with 6 water ligands.

When white light is passed through a solution of the Cu2+ ion, some of the energy is used to promote (or excite) an electron from an orbital in the lower group to an available orbital in the upper group. The energy that is absorbed is equal to the energy gap between the 2 groups.

In the case of the Cu2+ complex, yellow light is absorbed and as a consequence the other colours are transmitted and combine to give the solution a blue colour (the complementary colour of yellow). Skip flash movie


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The size of the energy gap between the 2 groups of d orbitals will vary with the transition metal ion, its oxidation state and the nature of the ligands. The further apart the groups are split, the greater the energy required to promote an electron (and the shorter the wavelength). Ammonia ligands, for instance, will cause greater splitting than water ligands.

In general, d-d transitions involve very low epsilonmax values of 1 to 102. This means that quantitative analysis is not all that useful because sensitivity is relatively poor.

Absorption of UV-Vis radiation by transition metal complexes will also be impacted by those factors previously discussed in relation to organic compounds (to affect lambdamax and epsilonmax). These factors are the choice of solvent and the concentration, pH and temperature of the sample. The analyst needs to ensure that these factors are controlled so that accurate and precise analytical outcomes are obtained.

 

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