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3.3: Hard and Soft Acids and Bases

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  • Lewis acids and bases can be classified by designating them as hard or soft.

    Hard Acids/Bases:

    "Hard" acids and bases have a high charge (positive for acids, negative for bases) to ionic radius ratio along with higher oxidation states. Hard acids are not very polarizable and have high charge densities. Thus metal ions with high positive charges and smaller ionic sizes tend to be hard acids. Early transition metal ions in the 3d series tend to be hard Lewis acids. Hard bases are typically small anions and neutral molecules. Some examples of hard acids and bases include: H+, O2-, OH-, F-, Fe3+, and Al3+.

    Soft Acids/Bases:

    "Soft" acids or bases have a low charge to radius ratio, with low oxidation states. They are normally larger ions that are polarizable. For example, I- and S2- are soft bases and low charge density transition metals, such as Ag+, are considered soft acids. Soft acids often include transition metals in the second and third row of the periodic table that have a +1 or +2 charge, as well as late transition metals (especially those in the 4d and 5d series) with filled or almost completely filled d orbitals.

    Acids and bases are not strictly hard or soft, with many ions and compounds being classified as intermediate. For example, trimethylborane, Fe2+, and Pb2+ cations are intermediate acids, and pyridine and aniline are examples of intermediate bases. An element can also change its hard/soft character depending on its oxidation state. The most extreme example is hydrogen, where H+ is a hard acid and H- is a soft base. Ni3+ (as in the layered compound NiOOH) is a hard acid, but Ni0 (as in Ni(CO)4) is a soft acid. The figures below show hard/soft trends for acids (left) and bases (right) in the periodic table. For bases, the major hard/soft discontinuity is between the 2nd row (N,O,F) and the rows below.

    For acids, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, In, Ta, W, Re, Pb, and Bi are intermediate acids. Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, Hg, and Tl are soft acids. All other acids are hard.

    Hard-soft trends for acids


    For bases, Cl is fully intermediate, S is between intermediate and soft, N is between intermediate and hard. O and F are hard bases. All other bases are soft.

    Hard-soft trends for bases

    Like binds with Like
    Hard acids interact more strongly with hard bases than they do with soft bases, and soft acids interact more strongly with soft bases than hard bases. Thus, the most stable complexes are those with hard-hard and soft-soft interactions. This tendency is illustrated in the table below, which shows the trend in formation constants for hard and soft acids. Hard acids bind halides in the order F- > Cl- > Br- > I-, whereas soft acids follow the opposite trend.

    Log K1 fluoride chloride bromide iodide acid classification
    Fe3+ 6.0 1.4 0.5 - Hard
    Pb2+ 1.3 0.9 1.1 1.3 Intermediate
    Ag+ 0.4 3.3 4.7 6.6 Soft
    Hg2+ 1.0 6.7 8.9 12.9 Soft

    The softest metal ion in the periodic table is Au+(aq). It forms stable complexes with soft bases such as phosphines and CN-, but not with hard bases such as O2- or F-. The affinity of Au+ for the soft base CN- is high, and the resulting [Au(CN)2]- complex is so stable that gold (which is normally very difficult to oxidize) can be oxidized by oxygen in the air:

    \[\ce{4Au_{(s)} + 8CN^{-}_{(aq)} + O2_{(g)} + 2H2O = 4[Au(CN)2]^{-}_{(aq)} + 4OH^{-}}\]

    This reaction is used in gold mining to separate small flakes of Au from large volumes of sand and other oxides. Ag is similarly dissolved by air oxidation in cyanide solutions. The precious metals are then isolated from the solution using chemical reducing agents or by electroplating. The use of cyanide ion on a large scale in mining, however, creates a potentially serious environmental hazard. In 2000, a spill at Baia Mare, Romania resulted in the worst environmental disaster in Europe since Chernobyl. Cyanide, which is highly toxic, is gradually oxidized by air to the less toxic cyanate (OCN-) ion. On the laboratory scale, cyanide plating solutions are typically disposed of by using bleach to oxidize CN- to OCN-, and the metal is recovered as an insoluble chloride salt.

    Netted solution pond next to cyanide heap leaching of gold ore near Elko, Nevada (1992).

    The Au3+ ion, because of its higher positive charge, is a harder acid than Au+ and can form complexes with harder bases such as H2O and amines. In keeping with the "like binds like" principle, the compound AuI (soft-soft) is stable, but AuI3 (hard-soft) is unknown. Conversely, AuF has never been isolated but AuF3 (hard-hard) is stable.

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