Hard Soft Acid Bases

• Hard Lewis Acid


  • H+
  • Li+
  • Na+
  • K+
  • Be2+
  • Mg2+
  • Ca2+
  • Al3+
  • Cr2+
  • Cr3+
  • Mn2+
  • Fe3+
  • Co3+
  • Ti4+
• Borderline Lewis Acid


  • Fe2+
  • Co2+
  • Ni2+
  • Cu2+
  • Zn2+
  • Pb2+
• Soft Lewis Acid


  • Cu+
  • Ag+
  • Au+
  • Cd2+
  • Hg22+
  • Hg2+
  • Hg+
  • Pd2+
  • Pt2+
  • Pt4+
  • Fe0
  • Ni0
• Hard Lewis Bases


  • F-
  • OH-
  • H2O
  • O2-
  • O2-
  • ROH
  • RO-
  • R2O
  • CH3CO2-
• Borderline Lewis Bases


  • Cl-
  • Br-
• Soft Lewis Bases


  • H-
  • R-
  • I-
  • C2H4
  • NH3
  • RNH2
  • N2H4
  • NO2-
  • N3-
  • N2
  • SO32-
  • H2S
  • HS-
  • S2-
  • RSH
  • RS-
  • R2S
  • S2O32-
• Other Lewis Bases


  • NO3-
  • ClO4-
  • CO32-
  • SO42-
  • PO43-
  • C6H5NH2
  • SCN-
  • Pyridine (C5H5N)
  • SCN-
  • CN-
  • RNC
  • CO
• The donor atoms of the base are highlighted with underline
• Hard acid


  • Small cations, not easily to polarized, high charged, low electronegativity for a metal
• Hard base


  • Halides, oxoanions (high charge/oxidation state), ionic bonding dominant, very high electronegativity, small sized donor atom
• Interaction between hard acid/base

  • Predominantly coulombic, ionic, dipole-dipole and electrostatic
• Dipole–dipole interactions are electrostatic interactions between molecules which have permanent dipoles - partial charged (e.g., HCl-HCl)
• Soft acid


  • Larger cations, easily to polarize, low charge, intermediate to high electronegativity
• Electric polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field
• Soft base


  • Large size, intermediate to high electronegativity