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(+/) effect and the resonance (-f-Æ) effect are operative in the case of the 4,4'-dimethyl dipyridyl ligand. A red shift  of the intra /—/ transitions in the Dimp-complex of Nd3+ and an enhancement of the fluorescence intensities  for the Eu3+ and Tb3+ chelates compared to the unsubstituted complexes are observed. These changes are explained by the +/ and -f-i2 effects.
1,10-phenanthroline complexes.—The honour of demonstrating the formation of phenanthroline (Phen) complexes of the rare earth in aqueous solution goes to Kononenko and Poluektov . Later Lobanov and Samirnova , and Hart and Laming  reported the isolation of solid complexes from ethanolic media. The following europium complexes are reported.
Compounds of Europium
Eu(Phen)2(H20)C13, Eu(Phen) (CH3COO)s, Eu(Phen)3(SCN)3 (Xm = 4662, & = 3.32), and Eu(Phen)2(NOs)3 (Xm = 4471, p = 3.36).
The far infrared spectra of the phenanthroline complexes show two distinct patterns  in the 240—170 cm-1 region. The lighter members, La to Sm, show spectra which have several strong to medium bands located from 175 to 220 cm-1 (Table 26). With Eu to Lu several strong bands are observed in the 235 cm-1 region.
Phthalocyanine complexes.—Frigerio  first prepared the rare earth phthalocyanines in 1962. Later Plytjshchev and Shklover [378, 379] investigated the Sm and Er phthalocyanine complexes in some detail. Rare earth phthalocyanines including europium were prepared [377~\ by reacting the anhydrous nitrate or chloride with either lithium or sodium phthalocyanine in an anhydrous solvent like dimethylformamide (DMF) or methylsulphoxide (MSO). It was claimed that at least one anion is directly bonded to the central rare earth. The reaction of sulphonated phthalocyanine with various rare earth chlorides and nitrates in an aqueous medium was also investigated by Frigerio  and water insoluble sulphonated phthalocyanin complexes were isolated. An excess of metal ions over that required by stoichiometry was used. In the sulphonated complexes the sulphonate groups were also found to act as coordination centers. However, replacement of the metal ions from the sulphonate groups was easily achieved.
2,2',2?-ter'pyridyl complexes.—Rare earth salts were found [380, 381] to react with terpyridyl in ethanol and the rare earth-mowo-terpyridyl salts were isolated in the solid state. For europium only a fcis-terpyridyl complex, Eu(Terp)2Cl3*4H20, can be isolated  by the reaction of the chloride with two equivalents of terpyridyl. It is quite remarkable that only europium produced a fo’s-complex. The wwmo-terpyridyl complex of europium has the formula Eu(Terp)Cl3 • H20.
The infrared spectra of the mowo-terpyridyl complexes have been analyzed. Sinha [367, 380—382] has found the “breathing” vibration of
Coordination Compounds Containing Organic Ligands
both dipyridyl and terpyridyl ligands to be particularly sensitive to the nature of the metal ion, and a shift of this vibration towards higher frequency due to coordination has been observed. The “breathing” vibration of terpyridyl occurs at 988 cm-1. Taking the shift of this vibration as a measure of metal-ligand interaction Sinha  has proposed the spectrochemical series for the rare earth-mono-terpyridyl complexes shown below.
Pr pa Nd fh Gd < Sm < Tb < Eu < Ho <Ce w Dy Er < Tm < Yb
B. Ligands Having Oxygens as Donors
Oxygen-containing organic ligands can best be classified according to the nature of the functional groups, i.e. as (i) acidic (COOH) (ii) hy-droxylic (OH) (iii) aldehydic (CHO) and (iv) ketonic (C=0). Ligands containing one or more of these functional groups can act as oxygen
donors. The carboxylate ^j ion is especially interesting as it can
behave either as a unidentate or a bidentate ligand. Although —OH, —CHO and C=0 groups use the lone pair electrons of the oxygen for coordination under certain conditions protolysis of the —OH group can occur.
Mono- and Di-Carboxylic Acid Complexes
Formates. — The simplest wiono-carboxylic acid is formic acid (HCOOH). Formate complexes have not been extensively investigated although Sabkar [383J has mentioned rare earth formates. The cerium group rare earths form spherulic formates which are hexagonal. This property is used to identify small amounts of these elements. These formates are prepared by dissolving freshly prepared rare earth hydroxides in formic acid.
Acetates. — Acetate complexes are more common and Sonesson [384— 386] has made an extensive investigation on the rare earth-acetate (M3+/ CHsCOO-) system. He has reported the stepwise stability constants (at /1 = 2), and it is evident from his investigations that at least three and possibly four acetate groups are coordinated to the central rare earth ion . The rare earth-acetate system was later reinvestigated by Kolat and Powell  at lower ionic strength (ju = 0.1). A comparison of the stability constant data (Table 27) at different ionic strengths shows that the values of the first stability constant, log ki, at ^ = 0.1 are quite large compared to those obtained at (jl — 2.0. However, the magnitude of log kz compares reasonably well for both ionic strengths. It is quite curious to see that Eu3+ has almost the same log value as Sm3+ con-