Metals in medicine
Metals in medicine

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Metals in medicine

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

Timing: Allow approximately 1 hour

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.

  • What is the prominent oxidation state of the lanthanides?

  • It is +3. Other states do exist: cap e times u super two postfix plus and cap s times m super two postfix plus, for example, and in aqueous solution cap c times e super four postfix plus can be formed.

Now take a look at the electronic configurations of the free lanthanide atoms and ions shown in Table 2.

Table 2  Electronic configurations of the free lanthanide atoms and ions.

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
  • What does the notation [Xe] represent?

  • 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.

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