. Author manuscript; available in PMC: 2010 Oct 1.

Published in final edited form as: Chem Rev. 2009 Oct;109(10):4921–4960. doi: 10.1021/cr900134a

Table 3.

Classification of Select Metal Ions and Donor Atoms According to Pearson’s HSAB Principle.

Hard Lewis Acids	Borderline Acids	Soft Acids
H⁺, Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Sc³⁺, Ti⁴⁺, Zr⁴⁺, Cr³⁺, Al³⁺, Ga³⁺, La³⁺, Gd³⁺, Co³⁺, Fe³⁺	Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Bi³⁺, Rh³⁺, Ir³⁺	Cu⁺, Au⁺, Ag⁺, Tl⁺, Hg⁺, Pd²⁺, Cd²⁺, Pt²⁺, Hg²⁺

Hard Lewis Bases	Borderline Bases	Soft Bases
F⁻, OH⁻, H₂O, ROH, Cl⁻, RO⁻, R₂O, CH₃CO₂⁻, NH₃, RNH₂, NH₂NH₂, CO₃²⁻, NO₃⁻, O₂⁻, SO₄²⁻, PO₄³⁻, ClO₄⁻	NO₂⁻, Br⁻, N₃⁻, N₂, C₆H₅NH₂, pyridine, imidazole	RSH, RS⁻, R₂S, S₂⁻, CN⁻, RNC, CO, I⁻, R₃As, R₃P, C₆H₅, C₂H₄, H₂S, HS⁻, H⁻, R⁻