Annis Coulton (Y13): Understanding Coloured Complexes of Transition Metals

For Year 13 students the last term has been focused on completing our IB courses and beginning revision. Our last topics in both Chemistry and Physics were key in tying together our study of these subjects over the last two years.

One of the final topics in IBDP Chemistry was related to coloured complexes of transition metals. Students of GCSE sciences will be familiar with the idea of colourful compounds being characteristic of the transition elements which are found in the centre of the periodic table and display unique properties combining tendencies of metals and non-metals. This topic enhanced our understanding of why these bright colours are displayed by the transition metal compounds, a complicated topic that I was fortunate enough to attend a taster lecture on at the Cambridge University Open Day.

We learned that as transition metals can have many unpaired electrons (which is fundamental to understanding their unique properties), they can accept electrons to bond to molecules called ligands, like water or ammonia, that surround the metal ion. The ligands cause a disparity in the energy of the different electrons, making it possible for some electrons to absorb specific wavelengths of light when white light (such as daylight) is incident upon them to gain the energy to be promoted to the higher of the disparate energy levels. This absorption causes the removal of a colour from the transmitted light, so the complex ion in solution appears to be the opposite colour on the colour wheel, or complementary colour, of that absorbed.

We studied the basics of this process in Year 12, and I completed my Internal Assessment (a piece of coursework for each IB course - typically a lab report in Chemistry and Physics) on an experiment which made use of this principle [pictured]. However, returning to it at the end of the course enabled us to develop our understanding by drawing links with the mechanism of the ligand bonding and the organic chemistry reaction mechanisms we studied more recently, as well as considering the geometry of the complex ions. The moment when different parts of a topic come together is, for me, the most satisfying experience of studying sciences, and I especially value the opportunity provided by the IB course and science GCSE programs of studying multiple sciences together and making cross-subject links.

Annis Coulton - Year 13 Science Prefect