First thing: they’re no longer the lanthanides and actinides. They were renamed by IUPAC.
Secondly: they’re in that rather neglected bit of the periodic table, that bit under the transition metals. They are part of the “f” block, in that the outermost electrons naturally reside in the f orbitals. Well known examples are Uranium and Plutonium. If you want some info about orbital theory and how chemists use them, this link is good.
We have much larger coordination numbers and much more fun, the 18 electron rule (an expansion of the octet rule) does not apply.
The chemistry of both is different because the 4f orbital behaves differently from the 5f orbital, so some subheadings for each are below:
Lanthanoids.
We call these the rare earth elements (along with scandium and yttrium of the actinoids) because we can find them in the earth. But they’re not actually *that* rare: quite a few have an abundance greater than gold and some greater than lead, for example. Cerium is the most abundant. You’ll find most of the world’s supply of rare earth metals in China and the US. They all occur naturally apart from Pm. From bastnäsite you can get the lighter lanthanoids, and you can get all of the metals (excluding Pm) from monazite. We can obtain 147Pm as a product of fission, including from nuclear reactors.
We use them daily in:
- catalytic converters (Cerium oxides)
- lasers (Neodymium)
- speakers (ta Rolphus! - Neodymium)
They’re pale and *glow* brightly under UV light, which is fun. They’re also paramagnetic.
It’s probably best to think them as “soft” s-block metals (Group 1 and 2) rather than transition metals. Because:
- Metal-ligand bonds aren’t really important compared to transition metal chemistry.
- The 4f-orbitals are deeply buried away (at least compared to the 5f orbitals)
- There’s also little preference in bond direction like the s-block.
- There’s also not much difference between members of the lathanoids compared to transition metals as well.
- The splitting of the degenerate set of f orbitals in crystal fields is tiny, so crystal field stabilisation effects don’t really matter like they do in transition metal chemistry.
So due to the above (but changing a +1 or +2 oxidation state to +3), we can assume…
- When the Lanthanides are exposed to air, they tarnish from their original silvery-white, forming their oxides - this is more pronounced as you go along the period.
- They are somewhat soft metals.
- They burn easily in air.
- They’re strong reducing agents
- Their boiling points and melting points are high
- All the Lanthanides are highly reactive.
- They react with Hydrogen in an exothermic reaction
- They react easily with most non-metals
- Preferences in coordination numbers and geometries tend to be controlled by steric effects (VSEPR)
The sorts of species lanthanoids like (hard to soft) are F-, OH-, H2O, NO3- and Cl-. Lanthanoid 3+ ions are “hard” themselves and thus prefer hard species to bond with over soft ones.
The ionic radii of the lananoids *decrease* as you go from left to right across the group, which is called the lathanoid contraction. This is because there is imperfect shielding of one electron by the others in the same sub-shell. Eu and Tb have slightly bigger radii as atoms, and Lanthanoids have smaller radii than Actinoids. Size is the most important factor in determining stability constants and coordination numbers of coordination complexes.
Lanthanoid and Halides
- Predictable. Ln + F2 —> LnF3.
- Can also get LnX4 from Ce, Pr and Tb.
- TbCl3 to LuCl3 are 6 co-ordinate, LaCl3 to GdCl3 are 9 co-ordinate
Lanthanoid and Hydroxides/oxides
- LnOH - strong base. strength of base + solubility goes down as you go across the series.
- Ln2O3 formed by thermal decomposition of oxoacid salts, but Ce, Pr and Tb give higher oxides and need H2 to reduce to Ln2O3.
Complexes of Ln(III)
Ln3+ ions are hard. Ln3+ in water tends to be 9 co-ordinate. (tricapped trigonal prismatic)
Spectroscopic stuff (if you don’t know about this area, don’t worry for now)
Spin orbit coupling is more important than crystal field splitting.
S,P,D,F,G terms are possible, with different values of J, so the number of possible transitions is *large*, so large number of possible absorptions.
Absorptions due to f-f transitions are sharp. Absorptions due to 4f-5d transitions are broad. Why?
- The 4f orbitals are well shielded from any ligand effects as they are so contracted, and therefore the f-f transitions are as sharp as they would be in atomic spectra.
- In contrast the d-d transitions in transition metal ions are typically broadened by interaction with ligands.
Actinoids tomorrow!
