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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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Coordination Compounds Containing Organic Ligands

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92 Compounds of Europium
There are, however, two strong pieces of evidence against such steric hirifctrance leading to change of dentate-character with the EDTA ligand. (1) Molecular models [448, 449] have shown that it is possible to arrange all six donor groups of EDTA around a relatively large cation without introducing excessive strain and still have room for at least one water molecule. (2) Recent calorimetric measurements [450] of AF°, AH° and AS° show the presence of two series based on the variation of both AH° and AS°. Since the entropy terms are usually small for the formation of simple complexes, a basic trend in AH° for the rare earth series is suggested [450], which is independent of the complexing agent. Moreover, a comparison of stability constants for EDTA with NTA shows the same trend and presence of two groups (incidentally, almost all of the rare earth complexes can in fact be divided into two groups). Since NTA has only four potential centers for coordination and shows discontinuity after gadolinium, it is very unlikely that the discontinuity in EDTA complexes is due to change of coordination character of the latter ligand.
Solid EDTA complexes have been isolated [33, 451, 452] and the nature of the bonding involved has been studied. Two types of solid complexes H[M(EDTA)] • and Mi[M(EDTA)] • 2/H2O (Mi = an
alkali metal or NH4) have been isolated.
Dehydration of the former at 110° C for 24 hr. results in two types of complexes with varying amounts of water [452]. Those of Ce, Pr, Nd and Sm retained one mole of water per mole of complex, H[M(EDTA)] • H2O, and a pentadentate nature of the EDTA was inferred form the vibrational spectra of the carboxylate ion (a strong infrared absorption band at 1670 cm-1 is observed indicating a highly associated COOH group). For heavier rare earths, i.e. from Eu to Lu, the same drying process yielded anhydrous complex, H[M(EDTA)]. The infrared spectra of these anhydrous complexes show no band in the 1670 cm-1 region indicating the absence of COOH group. A satisfactory explanation of the nature of the EDTA in these anhydrous complexes is not easily found. Kolat and Powell [452] assume the presence of a hexadentate ligand in the anhydrous complexes (Eu—Lu), the free proton being attached to a carbonyl group. Although the infrared data indicate a change of coordination character of the ligand in the solid phase such a change does not take place in solution. Apart from the foregoing disscussion of the stability constants and thermodynamic functions in solution, Kolat and Powell [452] provided additional support by measuring acid dissociation (pK) constants of the H[M(EDTA)] complexes. Their approximate values are
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