3.5 Chemistry of the lanthanides
It was mentioned in the previous section that complexes of gadolinium, a lanthanide, are used as MRI contrast agents. So, before continuing your study of MRI and contrast agents in particular, you’ll take a short diversion and look at some relevant background chemistry of the lanthanides.
But before getting started, here is a word about nomenclature.
Although we’re calling this series of elements the lanthanides, you’ll often come across the alternative – lanthanoid. In fact, it should be acknowledged that the International Union of Pure and Applied Chemistry (IUPAC) recommends using the latter as the ending ‘ide’ implies a negative ion – but lanthanide is still (arguably) the more commonly used name.
Start by working through the following exercise.
Activity 2 Exploring the lanthanides
To introduce you to the lanthanides and their chemistry, let’s start with the Periodic Table and think a little about where these elements fit – as you’ll see, it’s not entirely straightforward.
Work through the activity Exploring the lanthanides [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] , but please note that the first 48 seconds of the activity are not relevant for your study of this course. The final sentence also talks about actinides but, again, this is not relevant for your study.
When you have completed this activity, return here to continue your study.
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What is the prominent oxidation state of the lanthanides?
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It is +3. Other states do exist: and , for example, and in aqueous solution can be formed.
Now take a look at the electronic configurations of the free lanthanide atoms and ions shown in Table 2.
| Element | Symbol | Ln(g) | Ln2+(g) | Ln3+(g) |
|---|---|---|---|---|
| lanthanum | La | [Xe]5d16s2 | 5d1 | 4f0 |
| cerium | Ce | [Xe]4f15d16s2 | 4f2 | 4f1 |
| praseodymium | Pr | [Xe]4f36s2 | 4f3 | 4f2 |
| neodymium | Nd | [Xe]4f46s2 | 4f4 | 4f3 |
| promethium | Pm | [Xe]4f56s2 | 4f5 | 4f4 |
| samarium | Sm | [Xe]4f66s2 | 4f6 | 4f5 |
| europium | Eu | [Xe]4f76s2 | 4f7 | 4f6 |
| gadolinium | Gd | [Xe]4f75d16s2 | 4f75d1 | 4f7 |
| terbium | Tb | [Xe]4f96s2 | 4f9 | 4f8 |
| dysprosium | Dy | [Xe]4f106s2 | 4f10 | 4f9 |
| holmium | Ho | [Xe]4f116s2 | 4f11 | 4f10 |
| erbium | Er | [Xe]4f126s2 | 4f12 | 4f11 |
| thulium | Tm | [Xe]4f136s2 | 4f13 | 4f12 |
| ytterbium | Yb | [Xe]4f146s2 | 4f14 | 4f13 |
| lutecium | Lu | [Xe] 4f145d16s2 | 4f145d1 | 4f14 |
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What does the notation [Xe] represent?
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This is a shorthand notation to represent the filled shell of the preceding noble gas – in this case, xenon.
Note how the 4f shell progressively fills on moving across the series.
For free lanthanide atoms, the 4f electrons may be viewed as valence electrons. But in compounds, where two or more electrons are involved in bond formation, the residual 4f electrons experience increased nuclear charges and contract into the core.
In fact, in compounds in which the lanthanide has oxidation state +3, the lowering of energy is so marked that the 4f electrons may be classified as core electrons, so oxidation states higher than +3 are (almost) unknown.