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Table 24. Comparison of the stability constants of Eus+ and Am3+ sulphato complexes
M*+ I1 = = 1.0 &2 M*4 W II = 3 fe2
Eu 17 ± 1 50 H- O 37 ± 2 250 ± 30
Am 15 24 31 300
those for Am3+ although their ionic sizes are different. The values for EuSOI at pH 3 compare well with Sekine  data (EuSOj : log k\ = 1.54 ± 0.06; log = 2.69 ± 0.05).
The thermal decomposition of the rare earth sulphates, M2(SC>4)3, has been studied by Nathans and Wendlandt . The TGA curves show the reaction to take place in two steps. The first step corresponds to the formation of the oxysulphate, M2O2SO4, and in the second step decomposition of this takes place giving M2O3. The kinetics of the thermal
Inorganic Coordination Compounds 71
decomposition for La, Eu and Yb sulphates have been investigated and
M2(so4)3 - m2o2so4 - M2o3
the following activation energies (ZSact) for the sulphates and oxysulphates are reported.
м*+ J^act (kcal/mole) Sulphate Oxysulphate
La 62.2 ± 6.5 185 ± 33
Eu 58 ± 26 65 ± 12
Yb 46 ± 4 64 ± 19
Thiocyanate. — On the basis of /-orbital hybridization Diamond  predicted the formation of stronger actinide complexes with thiocyanate ion than for the rare earths. Subls and Choppin  have studied the ion exchange behaviour of many actinide and rare earth thiocyanate complexes and have shown that europium is eluted much sooner than americium from Dowex-1 with ammonium thiocyanate. The stability constants for the formation of MSCN2+ and M(SCN)£ complexes for Nd3+, Eu8+, Pu3+, Am3+, Cm3+, and Cf3+ have been measured  and are tabulated in Table 25. It is apparent from the table that the formation
Table 25. Comparison of the stability constant data of the Eus+ thiocyanate complex with that of Nd3+ and some actinides (n = 1.0 M at 25° C)
M3+ К fc2
Nd 6.47 ± 0.10 1.30 ± 0.20
Eu 5.05 ± 0.72 1.35 ± 0.20
Pu 2.90 ± 0.31 1.95 ± 0.29
Am 3.19 ± 0.10 2.2 ± 0.3
Cm 2.70 ± 0.19 2.6 ± 0.4
Cf 3.06 ± 0.18 —
constants of the rare earth (Nd3+ and Eu3+) thiocyanate complexes are higher than those of the actinide ones with comparable ionic radii. At present, there is no ready explanation for this anomalous behaviour.
Triphosphate.—Roppongi and Kato  investigated the rare earth-P3O10 system and reported the stability constants for all of them. The following values for the europium complexes were given, log &euhl = 4.90, log A;Eu(hl)2 = 8.68 and log = 16.91.
Tungstate. — McDonald et al.  synthesized europium tungstate, Eu2(W04)3, from EU2O3 and tungstic acid. The crystal structure of this compound was determined by Templeton and Zalkin . The
Compounds of Europium
crystals are found to be monoclinic belonging to the space group C2jc with a = 7.676 ± 0.003, b = 11.463 ± 0.003, c = 11.396 ± 0.005 A and /? = 109.63 ± 0.04°. Each europium atom has eight oxygen neighbours at an average distance of 2.43 A. The atomic arrangement is of the scheelite superlattice type with ordered vacancies in the cation positions. The atoms are however displaced by considerable distances from the scheelite locations.
Vanadates.—Milligan et al. [357,358] prepared the orthovanadates of some heavy metals including the rare earths by heating (750—1000° C,
2 hrs.) a mixture of metal oxide or oxalate and ammonium metavanadate. From their crystallographic studies they have shown that the orthovanadates, MVO4, of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, Lu and Y are tetragonal (Zircon type) and all isomorphous, having the following lattice constants.
MV04 Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
7.34 7.30 7.33 7.24 7.20 7.19 7.15 7.10 7.06 7.07 7.00 7.04 7.01 7.10 6.47 6.42 6.43 6.36 6.35 6.33 6.31 6.27 6.25 6.25 6.20 6.23 6.19 6.27
Slightly higher values are reported by Schwabz .
The solution chemistry of the rare earth vanadates has received considerable attention [359,360]. It has been found that when ammonium metavanadate solution is added to rare earth nitrate solutions, an increase in acidity results with the formation of the ortho vanadates (eq. 35). However, when the solutions are mixed in the opposite order the pH of the medium remains unchanged and precipitates of metavanadates the (eq. 36) are produced.
M(NOs)s + NH4VOs — MV04 + NH4NOs (35)
M(NOs)s + 3NH4VOs - M(VOs)s + 3NH4N03 (36)
This dependence on the order of mixing indicates an equilibrium of type
VO3 + H20 - HVOJ- + H+ (37)
existing in solution. A low concentration of rare earth ions favours the formation of meta vanadates, MfVOsJs, whereas at high concentrations the above equilibrium is shifted to the right and formation of orthovanadates, MVO4, results. Potentiometric titrations show the following values of pH for the precipitation of the metavanadates.
M*+ La Pr Nd Sm Eu Gd Dy Ho Lu Y
pH 5.40 5.10 4.90 4.70 4.45 4.40 4.80 5.30 5.70 4.80
Coordination Compounds Containing Organic Ligands
The metavanadates crystallize from aqueous solution as MV3O9 • 4H2O. If, however, solutions of ammonium metavanadate and rare earth nitrate are mixed with an equal volume of alcohol at a pH value depending on the ion (shown below) hexavanadates of the composition M4(Ve0i7)s • 48H2O are obtained.