Investigation of the Extraction Complexes of Light Lanthanides(III) with Bis(2,4,4-trimethylpentyl)dithiophosphinic Acid by EXAFS, IR, and MS in Comparison with the Americium(III) Complex

The structure of the extraction complexes of light lanthanides (La(III), Nd(III), Eu(III)) with bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HBTMPDTP) have been characterized with extended X-ray absorption fine structure
spectroscopy (EXAFS), IR, and MS; the IR spectrum of the extraction complex of 241Am with HBTMPDTP has
been studied too. The molecular formula of the extraction complexes of lanthanides is deduced to be HML4·H2O
(M = La, Nd, Eu; L = anion of HBTMPDTP). The coordination number of Ln(III) in the complexes is 8; the
coordinated donor atoms are 7 sulfur atoms from 4 HBTMDTP molecules and 1 O atom from a hydrated water
molecule. With the increase of the atomic number of Ln, the coordination bond lengths of Ln−O and Ln−S decrease
in the complexes. For La(III), Nd(III), and Eu(III), the coordination bond lengths of Ln−O are 2.70, 2.56, and 2.50,
respectively, the coordination bond lengths of Ln−S are 3.01, 2.91, and 2.84, respectively, and the average distances
between Ln and P atoms are 3.60, 3.53, and 3.46, respectively. The structure of the extraction complexes of Ln(III)
with HBTMDTP is different from that of the Am(III) extraction complex. The results of IR show that there is no
water coordinated with Am in the extraction complex. The molecular formula of the complex of Am(III) is deduced
as being HAmL4, and there are 8 S atoms from 4 HBTMPDTP molecules coordinated with Am. Composition and
structure differences of the extraction complexes may be one of the most most important factors affecting the
excellent selectivity of HBTMPDTP for Am(III) over Ln(III).