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Coordination Compounds Containing Organic Ligands
Citric, glycolic, a- hydroxy iso b uty ric, lactic, malic and tartaric acids have already been mentioned (p. 16).
With a-hydroxy acids chelation (Fig. 18) may help to form stronger complexes (higher k value) than for the corresponding monodentate
Fig. 18. Chelation in a-hydroxy acid complex.
ligands. The character of the bond between the metal and the oxygen of the a-hydroxy group will mainly depend on the nature of the metal ion. The more electropositive the metal, the stronger is this bond. This effect will consequently diminish the attraction or bond strength between the hydrogen and the oxygen of the OH-group.
Glycolates. — Glycolic acid (H2C(0H)C00H) is the simplest a-hydroxy acid. Jantsch and Grunkraut  obtained the glycolates (Glyc) of the lighter rare earths (La—Sm) as anhydrous, i. e. M(Glyc)s and those of heavy rare earths and Y as dihydrates, M(Glyc)3*2H20. Powell and Farrell  have recently reinvestigated the rare earth glycolate system and have studied solubilities.
Several investigations on the stability constants of the rare earth glycolate complexes have been made [404—407]. The stability constants for several rare earths including europium are presented in Table 28. It is quite evident by comparing the log k± values of acetates and glycolates that glycolate forms stronger complexes than acetate. The heavier rare earths are complexed to a much higher degree in glycolate complexes than in acetates.
Table 28. Stability constants of some rare eartha glycolate complexes 
M3+ log k± log k2 log kz
Ce 2.27 1.74 1.12
Sm 2.46 1.95 1.46
Eu 2.46 1.96 1.36
Gd 2.46 1.87 1.56
Ho 2.49 2.04 1.38
Yb 2.62 2.20 1.40
s Fora compilation of the stability constant data of other rare earths see ref. , p. 53.
6 Sinha, Europium
Compounds of Europium
Recently, Grenthe  compared the stability constants for the acetâte, glycolate and thioglycolate (mercaptoacetate) complexes of Eu3+ and Ams+ (/u = 0.5 at 20° C), and obtained the following log k\ values
Ligand Eu*+ Ams+
Acetate 1.94 1.99
Glycolate 2.57 2.82
Thioglycolate 1.55 1.55
It is apparent that Am3+ forms a stronger complex than Eu3+ with the same ligand. The main factors that may affect the stability of europium and americium complexes are ionic radii and the availability of the /-electrons. In the present case the /-electron participation of the 5/ orbital of Am3+ is possibly more important than the radius factor. If the radius factor was the more important one, Eu3+ with its smaller ionic size would be expected to form a stronger complex than Am3+. However, it is well-known that the 5/ orbitals are more polarizable than the wellshielded 4/ ones.
The changes in free energy, enthalpy and entropy for the formation of acetate, glycolate and thioglycolate complexes of La, Ce, Nd, Sm, Gd, Dy, Er, Yb and Y were determined by Grenthe . Comparison of these thermodynamic properties definitely indicates the formation of a chelate in the case of glycolate whereas thioglycolate acts as a mono-dentate ligand.
Diglycolates.—The rare earth-diglycolate IM3+/ 0<^_^ qqq ) sys^em
has been studied by Grenthe and Tobiasson . They compared the complexity constants of diglycolates (Table 29) with dipicolinates (p.91)and also studied the changes in free energy, enthalpy and entropy on the formation  of diglycolate and dipicolinate (DPA) complexes. They found some similarities, between these two ligands, but the dipicolinates were much more stable than the diglycolates (Table 29).
Methoxyacetates. — In view of the stronger complexation in glycolates compared to acetates one might expect the same behaviour for the methoxyacetate ion (CH3—O—CH2—COO -). However, the following log Jcn (ju. = 0.1, at 20° C) values for the rare earth-methoxyacetate complexes  demonstrate that except for the heavier rare earths
M3+ La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
log kx 2.03 2.06 2.07 2.11 2.13 2.12 2.06 2.05 2.05 2.07 2.08 2.08 2.08 2.09
log k2 0.85 1.0 1.18 1.23 1.26 1.30 0.95 0.95 1.08 1.15 1.15 1.15 1.28 1.28
Coordination Compounds Containing Organic Ligands 83
Table 29. Logarithm of first formation constant and the change of free energy (AG\), enthalpy (AH\) and entropy (AS\) of rare earth diglycolate complexes [409, 410]
M3+ log kx - AGl (kcal/mole) - AHi (kcal/mole) ASI (e.u./mole)
La 4.93 6.73 0.070 22.3
Ce 5.16 7.03 0.401 22.2
Pr 5.34 7.27 0.680 22.1
Nd 5.45 7.42 0.848 22.1
Sm 5.55 7.55 1.048 21.8
Eu 5.53 7.53 0.781 22.6
Gd 5.40 7.37 0.360 23.5
Tb 5.32 7.28 - 0.765 27.0
Dy 5.31 7.27 - 1.323 28.8
Ho 5.28 7.23 - 1.591 29.6
Er 5.34 7.32 - 1.660 30.1
Tm 5.49 7.52 - 1.574 30.5
Yb 5.55 7.60 - 1.423 30.2
Lu 5.64 7.71 - 1.230 30.0
(Dy—Lu) the complexity constants are smaller than for the corresponding acetate system. By virtue of the stronger electron withdrawing power (— I effect) of the methoxy group over hydrogen, methoxyacetic acid is rendered a stronger acid than acetic acid. Therefore, assuming that the bonding in these complexes is mainly through the carboxylate oxygen of the methoxyacetate ion, the apparent anomaly is explained. The steric effect of the methyl group in —OCH3 may also prevent the formation of a coordination bond via the methoxy oxygen. However, Powell et al.  believe that the methoxy group is weakly bonded to the rare earths. The author has constructed a Dreiding molecular model and found considerable steric hindrance due to the free rotation of the methyl group, which might be an important factor in solution.