7.1: Lanthanoids

The fifteen elements shown in Table \(\PageIndex{1}\) from lanthanum, La (4f⁰), to lutetium, Lu (4f¹⁴), are lanthanoids. Ln may be used as a general symbol for the lanthanoid elements. Although lanthanoids, scandium, Sc, and yttrium, Y, are sometimes called rare earth elements, they are relatively abundant in the earth’s crust. With the exception of promethium, Pm, which does form a stable isotope, even the least abundant thulium, Tm, and lutetium, Lu, are as abundant as iodine. Because lanthanoids have very similar properties and are difficult to separate from one another, they were not useful for basic research and application, and hence they were regarded as rare elements. Since a liquid-liquid solvent extraction method using tributylphosphine oxide became available in the 1960s, lanthanoid elements have been readily available and widely used not only for chemical research but also as materials in alloys, catalysts, lasers, cathode-ray tubes, etc.

Exercise \(\PageIndex{1}\)

What is the difference between lanthanoids and lanthanides?

Answer

Fifteen elements La-Lu are lanthanoids and fourteen elements Ce-Lu without lanthanum are lanthanides (meaning the elements similar to lanthanum). Occasionally the names are confused and 15 elements including lanthanum may be called lanthanides.

Table \(\PageIndex{1}\) Properties of lanthanoids

Atomic number	Name	Symbol	Electron configuration	M^3+ radius (pm)
57	Lanthanum	La	5d^16s^2	116
58	Cerium	Ce	4f^15d^16s^2	114
59	Praseodymium	Pr	4f^36s^2	113
60	Neodymium	Nd	4f^46s^2	111
61	Promethium	Pm	4f^56s^2	109
62	Samarium	Sm	4f^66s^2	108
63	Europium	Eu	4f^76s^2	107
64	Gadolinium	Ge	4f^75d^16s^2	105
65	Terbium	Tb	4f^96s^2	104
66	Dysprosium	Dy	4f^106s^2	103
67	Holmium	Ho	4f^116s^2	102
68	Erbium	Er	4f^126s^2	100
69	Thurium	Tm	4f^136s^2	99
70	Ytterbium	Yb	4f^146s^2	99
71	Lutetium	Lu	4f^145d^16s^2	98

Because the three stages of ionization enthalpy of lanthanoid elements are comparative low, they are positive elements and readily assume trivalent ionic states. Most compounds of lanthano other than Ce⁴⁺ (4f⁰), Eu²⁺ (4f⁷), Yb²⁺ (4f¹⁴), are usually Ln³⁺ ones. Ln³⁺ species are hard acids, and since f electrons are buried deeply and not used for bonding, they are hardly influenced by ligands. There is a tendency for atomic and ionic radii to decrease with the increase of the atomic number, and this phenomenon is called the lanthanide contraction. This contraction is due to small shielding effects of 4 f electrons, which causes the atomic nucleus to draw outer shell electrons strongly with an increase of atomic number.

Complexes of lanthanoid metals are 6 to 12 coordinate and especially many 8 and 9 coordinate compounds are known. Organometallic compounds with cyclopentadienyl ligands of the types \(\ce{Cp3Ln}\) or \(\ce{Cp2LnX}\) are also known, all of which are very reactive to oxygen or water.