## 2 Materials and Methods  
### Materials  
Phosphorus pentachloride (\(\geq\)98% pure, Acros Organics, Geel, Belgium) was purified by vacuum sublimation. Ammonium sulfate (\(\geq\)99% pure) and metallic sodium were obtained from Acros Organics and were used without further purification. Hexamethyldisilazane (HMDS), obtained from Penta-91, was purified under standard conditions and freshly distilled prior to use. 2-Ethylhexanol (99% pure, Acros Organics) was purified by distillation to remove water traces. 2,2,2-Trifluoroethanol (\(>\)95% pure, JSC \"PIM-Invest\", Moscow, Russia) was freshly distilled prior to use. Polyalkylphosphonitrile acid (PAPNA, \(\geq\)99.8% pure) and RED-3 (\(\geq\)98% pure, dearomatized extraction diluent, which is an oil hydrocarbon liquid of the C\({}_{9}\)-C\({}_{13}\) fraction along with several n-alkanes) was provided by D.I. Mendeleev University of Chemical Technology of Russia and used as received. Ethylenediaminetetraacetic acid disodium salt dihydrate (\(>\)98.5% pure, Sigma-Aldrich, St. Louis, MO, USA), ascorbic acid (99% pure, Sigma-Aldrich), xylenol orange (99% pure, JSC \"PCGroup\", Moscow, Russia) was used as received. The initial REE compounds were Y, Pr oxides (\(>\)99.5% pure, JSC \"Rushim\", Moscow, Russia), La, Ce, Nd, Sm, Gd, Dy, Yb nitrates (\(>\)99% pure, JSC \"Rushim\"), Ho chloride (\(>\)99% pure, JSC \"Rushim\"). Analysis of rare-earth elements for purity was carried out by comparing the results of titrimetric and gravimetric methods, as well as on Jobin Yvon 38P plasma spectrometer. Results of inductively coupled plasma spectroscopy analyses showed that the content of impurities in the REE salts did not exceed 0.01%. Corresponding metal oxides and chlorides were converted to nitrate form for extraction studies. The solvents, dried according to standard procedures, were used in a freshly distilled form. All glassware was dried in an oven or flame-dried under vacuum and cooled in the inert atmosphere before use. All reactions were performed using standard Schlenk techniques. **Equipment. ** The \({}^{31}\)P, \({}^{13}\)C, and \({}^{1}\)H NMR spectra were recorded on a Bruker AM300 spectrometer at 121, 75, and 300 MHz, respectively. Mass spectra of MALDI-TOF were obtained on the \"Bruker Auto Flex II\" instrument. Figure 1: Various complexes of {OP(OPh)\({}_{2}\)}\({}_{2}\)NH with Ba (**a**), Yb (**b**) and Hg (**c**). The content of impurities in metal salts was estimated by ICP spectra recorded on the ICP Jobin Yvon 38P plasma spectrometer. The content of functional acidic groups in extractants was determined by potentiometric titration using a pH meter Mettler Toledo S400. **Trichlorophosphazodichlorophosphonyl** (dichlorophosphinyl-phosphorimidic trichloride, TCDP) Cl\({}_{3}\)P=N-POCI\({}_{2}\) was synthesized by the reaction of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) with PCI\({}_{5}\) according to the procedure described in [39]. Purification was carried out by vacuum distillation, white crystals were obtained with T\({}_{\rm m}\) = 32-35 \({}^{\circ}\)C, 31P NMR (CDCl\({}_{3}\)): \(\delta_{\rm P}\) = \(-\)3.1 ppm (d, \(-\)Cl\({}_{3}\)P=N-), \(\delta_{\rm P}\) = \(-\)11.6 ppm (d, \(-\)P(=O)Cl\({}_{2}\)). Yield: 73%. **N-(trimethylsilyl)trichlorophosphoraniline** Cl\({}_{3}\)P=NSiMe\({}_{3}\) was synthesized as reported previously [40] by direct reaction of PCI\({}_{5}\) and hexamethyldisilazane. Yield: 52%. 31P NMR (CDCl\({}_{3}\)): \(\delta_{\rm P}\) = \(-\)54.8 ppm (s): 1H NMR (CDC\({}_{3}\)): \(\delta_{\rm H}\) = 0.16 ppm (d, J = 1 Hz); 13C NMR (CDCl\({}_{3}\)): \(\delta_{\rm C}\) = 1.9 ppm (d, J = 7 Hz, SiCH\({}_{3}\)). **Monochlorophosphazenc salt** (trichlorophosphazodichlorophosphonium hexachlorophosphate) [Cl\({}_{3}\)P=N-PCI\({}_{3}\)]\({}^{+}\)[PCI\({}_{6}\)]\({}^{-}\) was prepared by the reaction of Cl\({}_{3}\)P=NSiMe\({}_{3}\) with PCI\({}_{5}\).