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:
- Me = methyl
- R = alkyl
- Ar = aryl
- OTf = triflate