Transition metals - Colour of complex

One of the properties of transition metals is its ability to form complexes that are coloured. The rich colours of blue in copper (II) sulfate and deep purple in potassium permanganate are distinctive characteristics of a transition metal.

Hence, the property is a resultant of the definition that transition metals are elements that is able to form at least one ion or compound that has an incomplete d subshell.

Essentially, the colours which we see from transition metal complexes are due to an electronic transition. An electromagnetic wave in the UV-visible frequency was absorbed such that an electron can be transferred from a lower energy level to a higher energy level. The complementary colour would thus be reflected and it is this colour that we observed.

In addition, there are two routes for electron transfer. (1) d-d transition and (2) charge transfer.

1. d-d transition.

As most of the transition metal complexes (for example [Cu(H₂O)₆]²⁺) which you see is in an octahedral geometry, we will be using that as our starting point for the discussion of d-d transition.

The 6 ligands are placed on the x, y and z axis. Hence, this results in the d_{x² - y²} and the d_z² to receive the most electronic repulsion from the ligands, hence they are moved to a higher energy level.

This splitting of d-subshell is essential. There is now an energy gap between the lower set of d orbital and the higher set of d orbital. In addition, there must be an incompletely filled d orbital at the higher energy level so that an electron from the d orbital that has the lower energy level can be transferred.

An electromagnetic wave within the visible frequency is absorbed and that must correspond the the energy gap between the set of d orbital. This will result in the complementary colour to be reflected.

Do note that splitting of the d subshells to two different energy levels is still relevant in non-octahedral geometries (e.g tetrahedral). It is just that it may not be in this fixed arrangement of three d orbital are in the lower energy level and two d orbital are in the higher energy level.

2. Charge transfer.

Essentially, to enable d-d transition, there must be electrons in the 3d subshell. However, for potassium permanganate, the Mn is in +7 state. The electronic configuration of Mn (VII) is 1s²2s²2p⁶3s²3p⁶. There is no electron in the 3d subshell. Hence, the colour of potassium permanganate is due to charge transfer.

An electromagnetic wave within the UV-visible frequency is absorbed and that must correspond to the energy gap between a filled orbital of lower energy level and an empty orbital of higher energy level. This results in an electron to be excited from the lower energy level to the higher energy level, causing a complementary colour to be reflected.

The details to which electron is being excited and where it is excited to is beyond to scope of this discussion.

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^{Article written by Kwok YL 2010.}

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