Classification of Lewis Acids

Hard

Soft

Cu+, Ag+, Au+, Tl+, Hg22H Pd2+, Cd2+, Pt2+, Hg2+ Tl3 Au3+

H+, Li+, Na+, K+ Mg2+, Ca2+, Sr2+, Mn2+ Al3 La3+, Gd3+, Cr3+, Co3+, Fe3+, As3+ Si4+, Ti4+, Os4+ Borderline

Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Sn2+, Pb2+, Sb3+, Bi3+, Ru2+, Os2+, NO

For "class a" metal ions, the order of complex stability is as follows, whereas for "class b" metal ions the order is virtually the opposite.

In Table 10.3, metal ions that exhibit some importance in the bioinorganic chemistry are classified.

The general feature of a "class a" metal ions includes a small ionic radius, high positive charge, and are called "hard (Lewis) acids." "Class b" metal ions are in contrast associated with low oxidation state, large ionic radius, and are called "soft (Lewis) acids." This leads to a useful corollary, which is as simple and useful:

"Hard acids prefer to coordinate to hard bases, while soft acids prefer soft bases"

The stability order for soft acid complexes with Lewis bases is as follows:

For hard acids, the division is even sharper since only complexes with oxygen or fluorine donor atoms will exist in aqueous solution. The HSAB (hard and soft acid and base) principle will be widely applied in the following text.

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