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

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, 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: 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.

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.

OpenLearn - Metals in medicine
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