Examples of Hard and Soft Acids and Bases

Soft Lewis acids and bases are relatively large, polarizable atoms, ions, and molecules.

Hard Lewis acids and bases are relatively small and less polarizable.

In practice, soft acids prefer to associate with soft bases, and hard acids prefer to associate with hard bases.

List of Soft and Hard Acids (Metals)

Soft Acids

Ag+,   Au+,   Cd2+,   CdR+,   Co(CN)53-,   Cs+,   Cu+,   GaMe3,   GaCl3,   GaI3,   Hg+,   Hg22+,   Hg2+,   HgR+,   InCl3,   metal atoms,   Pd(PAr3)2,   Pd(PAr3)2 2+,   Pd2+,   Pt2+,   Pt4+,   Tl+,   Tl3+,   TlMe3

Borderline

Bi3+,   Co2+,   Cu2+,   Fe2+,   GaH3,   Ni2+,   Os2+,   Pb2+,   Rh3+,   Ru2+,   ZnR+,   Sb3+,   Sb3+,   Sc(OTf)3,   ScCl3,   Sn(OR)4,   SnCl4,   Yb(OTf)3,   YbCl3,   Zn2+

Hard Acids

Al3+,   Al(OR)3,   AlCl3,   AlH3,   AlMe3,   Be2+,   BeMe2,   Ca2+,   Ce4+,   Co3+,   Cr3+,   Cr6+,   Fe3+,   Ga3+,   Gd3+,   In3+,   InMe3,   Ir3+,   K+,   La3+,   Li+,   Lu3+,   Mg2+,   Na+,   Pu4+,   RMgX,   Sc3+,   Sn4+,   SnMe22+,   SnR3+,   Th4+,   Ti4+,   Ti(OR)4,   U4+,   UO22+,   VO2+,   Yb3+,   Zr4+

List of Soft and Hard Acids (Nonmetals)

Soft Acids

BH3,   Br,   Br+,   Br2,   carbenes,   chloroanil,   Cl,   HO+,   I,   I+,   I2,   ICN,   N,   O,   quinones,   RO+,   RO·,   RO2·,   RS+,   RSe+,   RTe+,   tetracyanoethyelene,   trinitrobenzene

Borderline

BBr3,   BR3,   C6H5+,   NO+,   R2C=NR,   R2CO,   R3C+,   RCHO,   SO2

Hard Acids

As3+,   B(OR)3,   BCl3,   BF3,   Cl7+,   CO2,   H+,   I5+,   I7+ ,   NC+,   R3Si+,   RCO+,   ROPO2+,   ROSO2+,   RPO2+,   RSO2+,   Se3+,   Si4+,   SO3

List of Soft and Hard Bases

Soft Bases

AsR3,   BH4-,   C6H6,   CN-,   CO,   H- ,   H2C=CH2,   I-,   P(OR)3,   PR3,   R-,   R2C=CR2,   RC≡CR,   R2S,   RNC,   RS-,   RSe-,   RSH,   S2-,   SCN-

Borderline

AlH4-,   Br-,   C5H5N,   C6H5NH2,   N2,   N3-,   NO2-,   R2C=NR,   R3N,   SO32-

Hard Bases

ClO4-,   CO32-,   F-,   H2O,   H2N-NH2,   NCS-,   NH3,   NO3-,   O2-,   OH-,   PO43-,   R2N-,   R2O,   RCO2-,   RNH2,   RO-,   ROH,   SO42-

Notes:

  1. Me = methyl
  2. R = alkyl
  3. Ar = aryl
  4. OTf = triflate