CHARACTERISTICS OF HARD, SOFT & BORDERLINE ACIDS & BASES
| Type of Acid/Base | CHARACTERISTICS | EXAMPLES |
| Hard acids | * Atomic centres of small ionic radii (<90 pm). * High positive charge. * Empty orbitals in their valence shells. * Low electronegativity (0.7-1.6) and low electron affinity. * Likely to be strongly solvated. * High energy LUMO. | H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, Sn2+ Al3+, Ga3+, In3+, Cr3+, Co3+, Fe3+, Ir3+, La3+, Si4+, Ti4+, Zr4+, Th4+, VO2+ , UO22+ BeMe2, BF3, BCl3, B(OR)3, AlMe3 |
| Soft acids | * Large radii (>90 pm). * Low or partial positive charge. * Completely filled orbitals in their valence shells. * Intermediate electronegativities (1.9-2.5) * Low energy LUMO's with large magnitude of LUMO coefficients. | Cu+, Ag+, Au+, Hg+ , Cs+ , Tl+ , Hg2+ , Pd2+, Cd2+ , Pt2+ Metal atoms in zero oxidation states |
| Border line acids | Fe2+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Pb2+ , B(CH3)3, SO2, NO+ | |
| Hard bases | * Small radii (around 120pm) & highly solvated . * electronegative atomic centres (3.0-4.0). * Weakly polarizable. * Difficult to be oxidized. * High energy HOMO. | H2O, OH-, F-, Cl-, CH3CO2-, PO43-, SO42-, CO32-, NO3-, ClO4-, ROH, RO-, R2O, NH3, RNH2, N2H4 |
| Soft bases | * Large atoms (>170 pm) with intermediate electronegativity (2.5-3.0). * High polarizability * Easily undergo oxidation. * Low energy HOMO's but large magnitude HOMO coefficients. | RSH, RS-, R2S, I-, CN-, SCN-, S2O3-, R3P, R3As (RO)3P, RNC, CO, C2H4, C6H6, R-, H- |
| Border line bases | Aniline, pyridine, N3-, Br-, NO2-, SO32-, N2 |
