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High oxidation states of rare earth elements
I was recently told that may of the rare earth elements can achieve high oxidation states greater then +3 (or in the case of cerium and terbium, +4)
but this isn't widely publicised. Is this true? Maybe the person who told me was confusing complexing with oxidation states. he was a mineral
collector after all, not a chemist.
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not_important
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In alkaline conditions, it's hard to keep cerium from going to the 4+ state. Moist Ce(OH)3 just sitting around in air darkens to yellow-orange and
then lightens some to yellow Ce(OH)4. Heat Ce(III) oxide, hydroxide, nitrate, oxalate, or similar compound in air and you end up with CeO2. Ceric
oxide is the cerium oxide polishing powder of commerce.
The sulfate Ce(SO4)2 and the double nitrate (NH4)2[Ce(NO3)6] - CAN - are used in chemistry as oxidisers.
Google "ceric"
http://en.wikipedia.org/wiki/Ceric
Terbium can be made to form TbO2, more easily the mixed (III,IV) oxide Tb4O7. Similarly praseodymium forms a mixed oxide Pr6O11 and can be forced to
form PrO2.
Europium, most easily, as well as samarium and ytterbium, can be had in the 2+ state. Running a solution of EuCl3 through a Jone's reductor into a
solution of a soluble sulfate with some sulfuric acid will precipitate EuSO4, similar to SrSO4 and BaSO4. If EuSO4 is boiled with concentrate Na2CO3,
EuCO3 is obtained. The Sm and Yb compounds are not stable to water, reducing it to form hydrogen.
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JohnWW
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That is correct. Those rare earth metals that can be obtained in the +4 oxidation state state are either those early in the series (Ce and Pr and just
possibly Nd) in which zero 4f electrons (or close to it) is the more stable configuration of cations (as well as utilizing the two 6s and one 5d
electron), or else those just after the midpoint of the series, like Tb, at which 7 (or close to it) 4f electrons is the more stable configuration in
the cations (with one unpaired electron in each of the 7 4f orbitals, giving a very high degree of paramagnetism and ferromagnetism). Of the
tetravalent metal cations, only Ce(4+) is stable in aqueous solution and forms a tetrafluoride, due to the electrons being less firmly held due to
the lesser nuclear charge. There is some possibility of Pr forming a +5 oxidation state, but this has not been confirmed.
Those rare metals which form stable +2 oxidation state compounds are those either just before the midpoint of the series like Eu and Sm, also leaving
7 (or close to it) unpaired 4f electrons, one in each 4f orbital, or those cations just before the end of the series like Yb and Tm with a completed
(or close to it) shell of 14 paired 4f electrons with very little or no paramagnetism.
For the same reasons, the rare-earth metals around Gd, in the middle of the series with 7 unpaired 4f electrons in the metallic state show the
highest degree of ferromagnetism of all metals, except for the metals around the middle of the homologous actinide metals such as Pu, Am, and Cm,
which have up to 7 unpaired 5f electrons. The latter metals are much more given to forming compounds with oxidation states greater than 3, up to +8 in
the case of Pu.
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And it's that ferromagnetism of Gd is what makes it one of my favorite elements (it's tied with Cs, Ga, W, Pu and U). So, Curium would in theory, be
the most Ferromagnetic metal because it has 7 unpaired 5f electrons, which are also very far away from the nucleus. Is Plutonium Ferromagnetic at all?
They say that Pu has the most complicated behavior of all metals because it passes through so many allotropic states within a narrow range of
temperatures and pressures. I would imangine that the electromagnetic properties vary a lot depending on the allotrope. I do not know much about the
properties of Curioum though.
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JohnWW
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Yes, plutonium is highly ferromagnetic (longest-lived isotope Pu-244, half-life 82 million years and found in trace amounts in pitchblende, although
Pu-239 is the most easily obtainable and that used in bombs, from spent enriched uranium reactor fuel rods in which U-238 has absorbed a neutron then
beta-decayed twice), and possibly more ferromagnetic than any of the lanthanide metals and certainly the "d" transition metals. Americium (used in
smoke detectors as Am-241, possibly obtained by bombarding Pu-239 with deuterons, although that is not the longest-lived isotope) and curium are
probably even more strongly ferromagnetic, but their intense radioactivity and cost very much limit possible usefulness. The extreme chemical toxicity
of plutonium, as well as its cost, limits the possibilities for its use in permanent magnets.
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