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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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k • 10-® 2.8 4.5 5.0 5.6 6.3
the dominating species is the [M(Cit)2]3- ion, whereas at low pH's other complex species like [HM(Cit)2]2-, [H2M(Cit)2]- and [HsM(Cit)2]0 are undoubtedly present in solution.
Gluconates. — Kostronina \145] investigated the formation of mono-and feis-gluconate complexes of the rare earths by a potentiometric method. The log k\ values obtained for these complexes (ju = 0.2 at 25° C) are as follows
M3+ La Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
log kx 2.32 2.60 2.66 2.75 2.74 2.66 2.47 2.40 2.42 2.50 2.80 2.85
The stability constants show the same trend as with acetate having an extended region (Eu—Ho) of the gadolinium break.
Salicylates.—In the course of his intramolecular energy transfer studies (p. 130) on europium chelates Weissman [630] prepared the 3-nitro and 5-nitro salicylate complexes. He was one of the first to demonstrate the phenomena of intramolecular energy transfer (IMET) from the coordinated ligands to the central rare earth ions giving characteristic fluorescence of the metal ions. However, no analytical data on these compounds are available.
Complexes containing Aldehydes and Ketones
Salicyladehyde complexes. — In addition to the salicylate complexes Weissman [630] also prepared a salicyladehyde (CaH^OHJCHO) complex of Eu3+ and studied the fluorescence properties of this complex. Weissman’s preparation was represented as a fris-salicyladehyde complex but no chemical analysis was given in support of this composition. Recently Charles [415] has prepared the salicylaldehyde complexes of La, Ce, Pr, Nd, Sm and Eu in excellent yields by the interaction of Na-
/CHO\
salicyladehyde ^CaH^^^ j with the respective metal chlorides. The
europium complex, EufC^HsCfeK was obtained in a 90 per cent yield. The infrared spectra of these complexes show a C=0 stretching fre-
Coordination Compounds Containing Organic Ligands
87
quency at ~ 1657 cm-1 which is independent of the nature of the rare earth ion. This behaviour is in strong contrast to that of the divalent metal salicylaldehyde complexes. The 3-nitro and 5-nitro salicylaldehyde complexes of Eu3+ were also investigated by Weissman [630].
Aceiylacetonates. — The (3-diketonate complexes of the trivalent rare earths are among the more stable of the complex species. The general
/H3Cv
I \c=o
name (3-diketones is given to acetylacetone I HC^f
\H3(X
derivatives. The acetylacetonate (Acac) complexes of some rare earths were prepared by Urbain [417] as early as 1896.
Extraction of the rare earths with acetylacetone has been investigated [418, 419] and is found to be enhanced by the decreasing basicity of the rare earth ions. The gas chromatographic separation of rare earth complexes with 2,2,6,6-tetramethyl-3,5-heptanedione has already been mentioned. The acetylacetonate complexes of the rare earths are reported to exist as either anhydrous [420, 421], mono- [422], di- [422] or trihydrates [422, 423]. Stites et al. [424] have studied the pH of the precipitation of several rare earth acetylacetonates and reported the melting points of the complexes. The europium acetylacetonate precipitated at pH 6.5, and melted at 144 —45° C. The existence of monomers and dimers for these complexes in nonaqueous solvents has been proposed [421, 425-427].
Stability constants in this system have also been investigated [428, 429]. The values [429] at 30° C in 0.1 M perchlorate media are given in Table 33. A change of 0.15 units in log ki values between consecutive
Table 33. Stability constants of the acetylacetonate complexes of the rare earths [429] (fi = 0.1 at 30° C)
M3+ log kx log k2 log ks
La 4.96 3.45 2.5
Ce 5.09 3.3 2.9
Pr 5.27 3.93 3.2
Nd 5.30 4.10 3.20
Sm 5.59 4.46 2.90
Eu 5.87 4.48 3.29
Gd 5.90 4.48 3.41
Tb 6.02 4.61 3.41
Dy 6.03 4.67 3.34
Ho 6.05 4.68 3.40
Er 5.99 4.68 3.38
Tm 6.09 4.76 3.48
Yb 6.18 4.86 3.60
Lu 6.23 4.77 3.63
and its various
88
Compounds of Europium
elements is evident in the La — Eu region. After Gd, however, the values remain fairly constant with a total change of only 0.2 units from Tb to Lu. It is interesting that the usual gadolinium break is not present except for a small discontinuity in Eu—Gd—Tb region. A comparison of instability constants [430] for a limited number of rare earth (La, Pr, Nd. Y) complexes with substituted (3-diketonates shows the following trend
acetylacetone < propionylacetone < benzoylacetone
Benzoylacetonate complexes. — The fm-benzoylacetonate complexe of the rare earths have been prepared by reacting sodium benzoylacetonate
Na+ ) with hydrated rare earth chlorides. They were isolated
as dihydrates. [636]
Recently Fry and Pirie [430a] reported that both heat and ultraviolet light decompose the fris-complex completely in neutral 3: 1 ethanol-methanol solvent. The decomposition products are mainly acetophenone and ethylacetate.
Dibenzoylmethide complexes.—Like the acetylacetonate complexes the
O o-
II I
dibenzoylmethide (DBM, C6H5C—CH=C—C6H5) complexes of the rare earths are usually obtained as hydrates or solvates. By slow addition of aqueous ammonia to a mixture of the rare earth chlorides and an excess of dibenzoylmethane in aqueous ethanol, hydrated complexes are usually obtained [431]. The products are invariably contaminated with free dibenzoylmethane, which however, can be readily extracted with cyclo-hexane [431]. Except for the Er, Yb and Lu the rare earth-DBM complexes form monohydrates M(DBM)3*H20. Desolvation with lighter rare earths is not easily achieved (see ref. [422] for closely related acetylacetonate complexes) due to a hydrolysis reaction (eq. (40)). The only
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