Lanthanides thiocyanates

Therefore the extent of extraction or back-extraction is governed by the concentration of X ia the aqueous phase, the distribution coefficients, and selectivities depending on the anion. In nitrate solutions, the distribution coefficient decreases as the atomic number of the REE increases, whereas ia thiocyanate solutions, the distribution coefficient roughly increases as the atomic number of the REE increases. The position of yttrium in the lanthanide series is not the same in nitrate and thiocyanate solutions, and this phenomenon has been used for high purity yttrium manufacture in the past. A combination of extraction by carboxyUc acids then by ammonium salts is also utilized for production of high purity yttrium. 

The reactions of lanthanide thiocyanates, nitrates, and chlorides with TPPO have been studied by Cousins and Hart (202, 203, 205). The reactions of lanthanide nitrates with TPPO in ethanol, acetone, ethylacetate and tetrahydrofuran are given in Fig. 1. The nature of the complexes isolated depends on the concentrations of the ligand and the metal ion, temperature of mixing, presence or absence of the seed of the desired complex, size of the cation, and the nature of the solvent. Tetrakis-TPPO complexes of Ce(III) and Nd(III) perchlorates have been reported (206, 207). Two of the perchlorates are coordinated to the metal ion in these complexes. 

Fig. 11.19 Distribution coefficients for lanthanide extraction with Aliquat 336 from nitrate (solid squares) and thiocyanate (solid circles) media. (From Ref. 1.)... 

Ion exchange techniques have been widely applied in the separation process. In the large-scale ammonium thiocyanate process, the metal is retained on strong base anion exchanger thus, separating it from the lighter lanthanide elements which are not strongly absorbed on the resin. 

Association between lanthanide ions and azide or thiocyanate ions has been studied in solution by electronic, Raman and NMR spectroscopy.186,187 The complexation constant in water between Nd3+ and N3 is approximately 2.5. Longitudinal relaxation time studies for Gd-Dy indicate that the M—N—NN angle is bent (135° approximately). 

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Anion sensing using visible-emitting lanthanide probes has proven successful (Tsukube et al., 2006) and this work is now being extended to Ybm probes, particularly for the detection of thiocyanate. The latter is the principal metabolite of cyanide anion and exists in human serum, saliva, and urine. The luminescent probe is a complex of hexadentate tetrakis(2-pyridylmethyl)ethylenediamine (tpen, see fig. 119) which bears two water molecules, [Yb(tpen)(H20)2](0tf)3. In absence of anion coordination, the 980-nm luminescence is quenched, but the replacement of the water molecules with thiocyanate or other anions such as acetate, nitrate or halogenides removes the quenching, which makes the complex a responsive probe. The largest effect (a six-fold increase in luminescence) is obtained for thiocyanate, followed by acetate and nitrate (3.5-fold) and chloride (two-fold). 

We have noted that N-bonded thiocyanate complexes dissociate readily in solution. Mixed ligand complexes of lanthanides with thiocyanates and L = ethanol, pyridine, triphenyl phosphorous oxide, phenanthroline, etc. have been synthesized. 

Lanthanide complexes of 1,10-phenanthroline (phen) have been obtained as chlorides, nitrates, sulfates, perchlorates acetates, thiocyanates and selenocyanates from various solvents. 

The ligand 1,2-bis(pyridine-a-aldimino) ethane has four nitrogen atoms for coordination and forms 1 1 solid complexes with lanthanide [246] chlorides, nitrates, thiocyanates or salicylates. Infrared spectra show evidence for nitrate coordination and pyridyl coordination. Attempts to isolate bis complexes were futile although there is spectral evidence for their existence in solution. 

A number of chloride complexes include Pr(terpy)Cl3.8H2O X-ray diffraction established the presence of a [Pr(terpy)C1(H20)5] + ion. Lanthanide nitrates react with 1 mole of terpy in MeCN to give [La(terpy)(N03)3(H20) ] (Ln = La, = 2 Ln = Ce-Ho, n = 1 Ln = Er-Lu, n = 0) with coordination numbers decreasing from 11 (La) to 9 (Er-Lu) if these are crystallized from water [Ln(terpy)(N03)2(H20)4] NO3 (Ln = La-Gd) and [Ln(terpy)(N03)2(H20)3]N03.2H20 (Ln = Tb-Lu) are formed these have both biden-tate and ionic nitrate and nine- and eight-coordinate lanthanides, respectively. Thiocyanate complexes Ln(terpy)2(NCS)3(Ln = Pr, Nd) are also nine coordinate. 

Question 4.8 Lanthanide thiocyanate compiexes are N-bonded, not S-bonded. Why is this... 

Eermium coprecipitates with lanthanide fluorides and hydroxides, showing it to form lanthanide-like Em + ions. These elute from cation-exchange resins slightly before Es +, whilst its chloride and thiocyanate complexes are eluted from anionic exchange resins just... 

Unusually, chloride is foimd in the first coordination sphere of lanthanum as well as nitrate in the serendipitously discovered mixed anion complexes [EaCl2(N03)(12-crown-4)]2 and [LaCl2(N03)(18-crown-6)]. A few complexes have been reported with other halides, such as [Sml3(dibenzo-18-crown-6)] (tricapped trigonal prismatic) and [LaBr3(12-crown-4)(acetone)] (distorted square antiprismatic). Lanthanide thiocyanate complexes of crown ethers are now starting to be studied. Several thiocyanate complexes of the... 

The thiocyanate complexes Ln(phen)3(NCS)3 (Ln = Pr, Nd) and Pr(bipy)3(NCS)3 are all nine-coordinate monomers. A considerable number of bipy (and phen) adducts of lanthanide dithiocarbamates [Ln(S2CNR2)3(L)]... 

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