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Europium - Sinha S.P.

Sinha S.P. Europium - Springer-Verlag, 1967. - 88 p.
Download (direct link): europium1967.djvu
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HOOC H2Cv /CH2 COOH
>N CH2 CHa— O—CHa CH,,—O—CH2 CH2 N<
HOOCH sy XIH2 COOH
EGTA
HOOC HgC. /CH2 COOH
>N CH2 CH2—O—CH2 CH2 N<
HOOC H2CX XJH2 COOH
EEDTA
•%
and mercury electrode methods. The results of mercury electrode measurements are given in Table 34 and are slightly lower than the results obtained by polarography.
The presence of the gadolinium break has led chemists to investigate the thermodynamics of complex formation and most of the work has been concentrated on EDTA and related ligands. The change of enthalpy (Al/?) and entropy (A&J) for several aminoacid chelates, EDTA [450], HEDTA [453]t DCTA [458], DTPA [460], NTA [467] and DPA [410] are collected in Table 37. The A/7° values for the heavy rare earths are smaller than for the lighter ones, and if the stabilization of the halffilled 4/-shell in Gd3+ was the dominating factor, one might expect a lower value of A//J for the Gd3+ complex than for the heavier rare earths. However, as this is not the case, the crystal field stabilization due to the half-filled shell cannot possibly be the main factor causing the gadolinium break.
96 Compounds of Europium
It will be noticed (Table 37) that the entropy of formation of the arrilnoacid complexes are quite large and again on the basis of A/Sj values the complexes could be divided into two groups. Thus, at present, there seems to be no simple explanation of the gadolinium break.
Eckhabd and Holleck [464] have reported the formation of Eu2 h complexes with EDTA and DCTA. They have shown the species [EuL]2-, [HEuL]- and [H2E11L], where L is a ligand, to occur in solution. The following log k values are reported.
Complex log*
EDTA DCTA
[EuL]2" 7.7 10.2
[HEuL] - 2.6 3.1
[H2EuL] 1.32 1.65
These log k values compare well with log k values for respective barium complexes.
Complexes with IMDA and related ligands. — A series of three substituted aminoacetic acids, IMDA, HIMDA and NTA constitute another set of very interesting ligands. The stability constants of the iminodiacetic acid (IMDA) [465], N-hydroxyethyliminodiacetic acid (HIMDA) [466] and nitrilotriacetic acid (NTA) complexes are compared in Table 34. Based on log k\ values the following trend of stability is noted. The
NTA > HIMDA > IMDA
N (CH2COOH)a HO CH2 CH2 N (CH2COOH)2 HN (CH2COOH)a
log ki values for the NTA complexes show a steady increase through the series whereas the log k% values increase from La to Dy and then decrease. HIMDA complexes are about 2 log k units more stable than IMDA complexes indicating the possibility of coordination through the hydroxyl group in HIMDA. Whereas both log ki and log k% values show a break at gadolinium for IMDA complexes, only a break of log k\ value at gadolinium is noted for HIMDA complexes. The log kz values of the latter increase from La to Dy and then decrease up to Yb. The log &2 value of Lu is higher than that of Yb. Both the HIMDA and NTA complexes have unusually large log k% values.
Dipicolinaies. — The rare earth-dipicolinate (DPA) system was mentioned in connection with the diglycolates. The stability constants \468] are givenin Table 34, and the AZ/J and A/S?values [410] arepresentedinTable37. The gadolinium break is also observed in the Eu—Dy region for log ki whereas a steady increase of log kz values is apparent throughout the series. The break in log kz values occurs at Ho rather than at Gd, and a decreasing tendency follows throughout the rest of the series.
Coordination Compounds Containing Organic Ligands 97
Table 37. Comparison of AH\ and AjSJ of some amino acid complexes of the rare earths
M3+ EDTA [450] A HI A S° (kcal/mole) (e.u./mole) HEDTA [453] AHl A/S® (kcal/mole) (e.u./mole) DCTA [458] A HI A£° (kcal/mole) (e.u./mole)
La -2.93 59.7 -2.20 54.2 + 3.55 86.7
Ce -2.94 60.8 -3.06 54.4 - -
Pr -3.20 61.4 -4.45 52.0 +5.00 95.6
Nd -3.62 61.3 -4.25 53.8 + 5.00 97.6
Sm —3.35 64.4 -4.65 54.4 + 5.00 102.0
Eu -2.56 67.6 -4.81 54.1 + 5.54 104.5
Gd -1.73 71.2 -4.66 54.1 + 5.75 105.3
Tb -1.11 75.5 -3.39 58.8 +5.00 105.1
Dy -1.21 77.3 -2.12 62.8 + 3.09 100.5
Ho -1.36 78.0 -1.14 66.3 + 1.18 95.0
Er -1.71 78.3 -0.32 69.4 +0.12 92.8
Tm -1.87 79.1 +0.92 74.5 -1.59 88.3
Yb -2.31 79.2 + 0.36 74.0 -4.49 80.2
Lu -2.51 79.1 +0.22 73.4 -4.92 79.2
M8+ DTPA [460] A HI AS® (kcal/mole) (e.u./mole) NTA [467] ah; asi (kcal/mole) (e.u./mole) DPA [410] A H\ A S\ (kcal/mole) (e.u./mole)
La -5.7 70.0 2.05 54.3 -3.13 25.8
Ce — - 1.25 53.8 —3.55 26.1
Pr -7.1 72.0 0.45 52.2 -3.91 26.1
Nd -5.8 79.4 0.68 53.8 -4.01 26.5
Sm -8.2 74.7 0.40 54.1 -4.28 25.9
Eu -8.1 75.3 0.93 55.8 -4.07 26.6
Gd -7.5 77.6 1.02 56.2 -3.58 27.8
Tb -7.7 78.1 1.71 58.8 —2.69 30.5
Dy -8.0 77.6 2.25 61.3 -2.17 32.5
Ho -7.6 78.7 2.63 63.3 -1.95 33.2
Er -7.3 79.6 2.51 63.5 -1.85 33.8
Tm —5.5 85.5 1.90 62.3 -1.83 34.2
Yb -5.5 85.1 2.09 63.7 -1.93 34.0
Lu -4.6 87.2 1.98 63.8 —2.19 33.9
8-quinolinol and substituted quinolinol complexes. — Charles and Per-rotto [469] have prepared rare earth-8-quinolinol complexes by a homogeneous pricipitation technique. They found that the metal : ligand ratio varied from 2.2 to 2.8, and it is actually quite difficult to achieve 1:3 stoichiometric composition. All rare earth-8-quinolinol complexes like the transition metal-8-quinolinol complexes show a new infrared band at which seems to be characteristic of quinolinol complexes.
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