SOLVENT-EXTRACTION OF DIVALENT METALS AS DIHYDROBIS(1-PYRAZOLYL)BORATE CHELATES

The extraction behavior of divalent metals (M2+) with dihydrobis(1-pyrazolyl)borate ligand (H2B(pz)2-) has been studied. Divalent transition metals form [M{H2B(pz)2}2] chelates readily and are extracted into the organic phase. The extraction constants of the first-row transition metals increase in the order Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II), which is in accordance with the Irving and Williams' order for complex stability. Although Be2+ is slightly extracted by H2B(pz)2-, Mg2+, Ca2+, Sr2+, and Ba2+ are not extracted, whether a synergistic agent such as trioctylphosphine oxide and tetrabutylammonium chloride is added or not. The characteristics of H2B(pz)2- in the extractability of metal ions are ascribed to that the basicity of coordinating nitrogen atoms is increased by negative charge of the molecule and inductive effect of the boron atom, and that adduct formation is restricted by the steric hindrance.