Investigation of the aggregation of the neodymium complexes of dialkylphosphoric, -oxothiophosphinic, and -dithiophosphiic acids in toluene

Small angle neutron scattering and visible absorption spectroscopy were used to study the neodymium complexes of a series of acidic organophosphorus extractants in deuterated toluene at high (≥ 33% theoretical capacity) metal loading. Organic phases containing 0.10M bis(2-ethylhexyl)phosphoric acid (HDEHP), bis(2,4,4-trimethylpentyl)oxothiophosphinic acid (HC302), or bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301) were used. Under these conditions, the neutron scattering experiments show that HDEHP and HC302 exist as dimeric species in the absence of Nd. Extraction of Nd disrupts the dimeric structure of the extractants to form dinuclear complexes with the formula Nd₂(DEHP)₆ or Nd₂(C3O2)₆. In contrast, 0.10 M HC301 is not dimeric in deuterated toluene. At 86% of the theoretical Nd capacity, the extracted complex is also dinuclear with the average formula Nd₂(C301)₆, but at 46% of the theoretical Nd capacity only the mononuclear complex, Nd(C301)₃, is formed. The Nd hypersensitive transitions in the visible region of the spectrum provide further support for this interpretation of the neutron scattering data and the equivalence of the Nd coordination in highly loaded HDEHP and bis(2,4,4-trimethylpenthyl)phosphinic acid.