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A generalization of the relative affinities of ligand atoms for metal ions on complex formation was made independently by Carleson and Irving , and by Ahrland and Larsson  in 1954. Later Ahrland, Chatt and Davies  reviewed the subject and proposed two classes (a and 6) of acceptors depending on the difference between their coordinating affinities with donor atoms. With class (a) acceptors their coordinating affinities towards the ligand atoms follow the sequence
F > Cl > Br > I
O > S > Se > Te
Within the same period, say Period II, the relative coordinating affinities of class (a) acceptors decrease as follows
F > O > N
and a reversal of this trend is observed in the case of the class (6) acceptors.
Rare earths belong to class (a) of the Ahrland, Chatt and Davies classification , but complexes where oxygen acts as donor atoms
48 Compounds of Europium
are particularly characteristic of them. It was not until recently that complexes of the rare earths with nitrogens as donors were reported .
In aqueous media the trivalent rare earth ions are strongly hydrated, and the formation of an aquo complex [M(OH2)»]3+ (where n is larger than six, perhaps eight or nine) takes place. There is also a distinct lowering of pH on dissolving the salts of rare earths in water. The extent of lowering of pH depends essentially on the concentration of the salt and the nature of the particular rare earth ion. The heavier rare earth ions which possess small ionic radii show a greater tendency to hydrolyse. Certain anions like the halides, sulphates and nitrates tend to form ion-pairs in aqueous solution. There is, however, spectroscopic evidence  that the formation of an ion-pair readily takes place in an alcoholic medium also.
The formation of complexes is affected by many physical and chemical factors. Such environmental factors as solvent, temperature and pressure are often important. Concentration factors sometimes markedly influence the stabilities of the complexed species in solution. The role of the donor atoms of the ligand in forming complexes has already been mentioned.
It would be expected that the lanthanide contraction (p. 30) would affect the stability constant1 (eq. (20)) of the complexes within the rare earth series. Whereas there are only slight departures from essentially monotonic variations of the ionic radii in the series with increasing Z, the stabilities of the rare earth complexes show a distinct irregularity or break near gadolinium. Chemists often refer this irregularity as the gadolinium break. It will be seen later that the region of the gadolinium break may extend from Sm to Dy. At present there is no simple explanation for this anomalous behaviour occuring in this region.
Both a review  and a monograph  describing various aspects of the complex chemistry of the rare earths recently been published. For convenience this chapter is divided into two parts, Part I dealing with the purely inorganic compounds and Part II dealing with complexes containing organic ligands. The inorganic compounds are arranged more or less alphabetically, and the complexes involving organic ligands are classified according to the nature of the donor atoms in the ligand.
1 The consecutive or stepwise formation (stability) constants kn are expressed as
MLn-i “I- I* ML»
_ [MLn] (20)
Often the logerithms of kn values are reported in the literature. Some workers prefer to report the instability constants, fcinst, rather than formation constants, where A:inst is the reciprocal of the formation constant.
Inorganic Coordination Compounds 49
Inorganic Coordination Compounds
Amide. — It has been pointed out before that europium behaves more or less like the alkaline earths and is closely related to strontium and barium. It is found to react with liquid ammonia at —78° С in much the same way as the alkali metals forming a characteristic deep blue solution. Eu(NH2)2 can be isolated  from the blue solution. Recent electron paramagnetic studies [26T\ of solutions of europium in liquid ammonia showed the presence of complex hyperfine lines arising from Eu2+ (®$7/2, g
— 1.990 ± 0.002) besides the characteristic single line of the solvated electron (g = 2.0014 ± 0.0002)1. The departure of the Eu2+ g-value from the free electron value is explained as being due to spin-orbit coupling and a slight admixture (3.6%) of the 6P7/2 state.
The МС1з—NH3 system (where M = La and Eu) has been investigated by Hutttg and Dauschan . In the case of Lads the presence of complexes with 8, 6, 4 and 2 moles of NH3 has been demonstrated. EuCls was found to form a decaammine complex.
Arsenate. — The arsenates of the rare earths crystallize  in two structural types, the huttonite and the zircon. The structural change from huttonite (La—Nd) to zircon (Sm—Lu) occurs at samarium. The lattice parameters of EuAsC>4 are a = 7.167 and с = 6.374 A. The rare earth arsenates can be prepared by reacting the nitrates with (NH^HAsC^, and heating the product to ~700° C.