Magnetic Properties of the Transition Elements

Magnetism is a funny thing isn’t it? When we experience two magnets repelling each other it’s a strange sensation. We can feel this spongy, springy force yet can’t see anything. Magnetism is a force we are exposed to everyday of our lives without really being aware of, however it has allowed life (as we know it) to evolve on the Earth. I’m talking about the Earth’s magnetic field and how it protects us from the harmful radiation of the sun. If you are lucky enough to have seen the Northern or Southern lights you are seeing the Earth’s magnetic field in action.

Magnetic properties of the transition elements are now covered as part of the Chemistry course, in the HL section 13.1 (first row Transition Elements) and the core option on Materials (option A) in section A2 ‘metals and inductively coupled plasma (ICP) spectroscopy.

The type of magnetism covered in the course is not the familiar type of magnetism. By this I mean permanent magnetism or ferromagnetism. This is what you find in Iron (and Nickel and Cobalt – all three lie next to each other in the periodic table – there must be a connection here).

The course covers two types of magnetism known as paramagnetism and diamagnetism. Put simply, a paramagnetic elements can be weakly magnetic in an external magnetic field. In other words, they are not permanently magnetic but will behave as weak magnets in magnetic fields (much like a steel bar can become magnetised in an electric field – in other words, an electromagnet)

Diamagnetic substances are not magnetic at all – in fact, they are weakly repelled by a magnetic field.

So what determines if an element is paramagnetic or diamagnetic?

The answer is quite simple. Any unpaired electrons will cause the material to be paramagnetic. And the more unpaired electrons there are, the more stronger it becomes.  This is because the unpaired electron (which is negatively charged and spinning) creates a small magnetic field itself. This property is also exploited in NMR.

If all the electrons are paired, the effect of the spinning is cancelled out – so the small magnetic field is not created. This is also why NMR detects elements with odd proton numbers (it’s not to do with the number of protons but the number of electrons).

Now, here is the interesting part. Some transition metal complexes can be paramagnetic or diamagnetic. The type of ligand can cause this effect to happen and it is handy to use your data book at this point to look at the Spectrochemical Series.

A ligand such as CO will cause the d orbitals in transition element to split a greater distance than say an OH^– ligand. This means that in a transition metal complex with OH^–, it is much easier for electrons to be promoted to higher split d orbitals as they are not as far apart as in CO.

When the electrons are promoted they are likely to be unpaired. So an element such as iron (with 6 d electrons) will have its electrons all paired in the three lower energy d orbitals with CO as a ligand, as opposed to when OH^– is a ligand where one or two d electrons may be promoted to the higher level d orbitals (and hence, become unpaired) as they are relatively closer together than with CO.

This would mean the OH^– complex would be weakly magnetic in a magnetic field where as the CO complex would not.

In the real world, this does property does not really affect things but it can be exploited in super conductors and it is for this reason that super conductor research (the search for high temperature superconducting materials)focuses on this property of transition metal complex ions.