Separation of first-series transition metal ions {Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)} and Cd(II) has been studied by solvent extraction using [HB(pz)₃]⁻ and [B(pz)₄]⁻. These ligands quantitatively extract all the studied metal ions. [HB(pz)₃]⁻ is one of the most powerful extractants of these metal ions, since it extracts them from acidic aqueous solution (pH 2.1). [B(pz)₄]⁻ extracts large metal ions, such as Mn(II) and Cd(II), around higher pH compared to [HB(pz)₃]⁻. Also, the extraction constant of [B(pz)₄]⁻ for Cu(II) is low and does not conform to the order of the Irving–Williams series. The different selectivity of these ligands cannot be explained on the basis of the basicity of the donor atoms. To explore the origin of the selectivity, X-ray crystal structures of the six extracted species have been determined. The complexes of [HB(pz)₃]₂Mn (1), [B(pz)₄]₂Mn (2), [B(pz)₄]₂Co (3), [B(pz)₄]₂Ni (4), and [B(pz)₄]₂Zn (6) have distorted octahedral geometries with each ligand having tridentate coordination. The complex [B(pz)₄]₂Cu (5) has square bipyramidal geometries. Examination of the dimensions of the above-mentioned complexes and the eight related ones has revealed that although the [HB(pz)₃]⁻ complexes are highly C₃ symmetric, the [B(pz)₄]⁻ complexes for Mn(II), Cu(II) and Cd(II) are largely distorted. These results suggest that [HB(pz)₃]⁻ is sterically efficient, and the increase in steric energy with complex formation is sufficiently low. On the other hand, because of the intraligand contact between pyrazolyl rings, [B(pz)₄]⁻ accompanies substantial steric energy increases in coordination to Mn(II), Cu(II) and Cd(II), decreasing the stability of the complexes and resulting in unique selectivity.