Selectivity of [HB(3,5-Me(2)pz)(3)](-) (pz = 1-pyrazolyl) for group 2 metal ions has been studied by liquid-liquid extraction and compared with that of [HB(pz)(3)](-) and [B(pz)(4)](-). Although all the extracted species of Mg2+, Ca2+, Sr2+ and Ba2+ are distorted octahedral A(2)M (A = poly(pyrazolyl)borate), the selectivity is highly dependent on the ligand. The selectivity of [HB(pz)(3)](-) is in the order Mg2+ > Ca2+ > Sr2+, [B(pz)(4)](-) is specific to Mg2+ and hardly extracts the larger metal ions. [HB(3,5-Me(2)pz)(3)](-) shows the selectivity order Ca2+ > Mg2+ > Sr2+ > Ba2+. The steric properties responsible for the selectivity have been elucidated by X-ray diffraction, H-1 and C-13 NMR, and molecular mechanics calculations. The results confirm the following conclusions : (i) the selectivity is principally governed by the stability of A(2)M complexes; (ii) the stability of [B(pz)(4)](2)M markedly decreases with the increase in ion size because of the intraligand contact between pyrazolyl rings; (iii) the 3-methyl group of [HB(3,5-Me(2)pz)(3)](-) stabilizes the complex with a large metal ion through shielding effects and destabilizes the complex with a small metal ion through interligand contact. For the very small ion Be2+, which takes tetrahedral geometry, the structures of extracted species are distinct. With [HB(3,5-Me(2)pz)(3)](-), [HB(3,5-Me(2)pz)(3)]BeOH is the dominant extracted species, which cyclizes to [HB(3,5-Me(2)pz)(3))BeOH](3) in diethyl ether. In contrast, [B(pz)(4)](-) forms [B(pz)(4)]Be-2, and [HB(pz)(3)](-) forms [HB(pz)(3)]Be-2 and [(HB(pz)(3))BeOH](3) depending on the initial concentration of Be2+. Structure variation in Be2+ complexes also arises from the intra- and interligand contact and shielding effects.