Hard-Soft Acid-Base Theory

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Hard acids or bases are compact, with the electrons held fairly tightly by the nucleus. They are not very polarizable. F- is a hard base, and metal ions such as Li+, a hard acid.

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Hard and Soft Acids & Bases

Large, highly polarizable ions are categorized as “soft.” Iodide is a soft base, and transition metals with low charge density, such as Ag+, are considered to be soft acids.

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Hard and Soft Acids & Bases

Hard acids tend to bind to hard bases.

Soft acids tend to bind to soft bases.

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Problem

Predict the solubility (high or low) of silver fluoride, silver iodide, lithium fluoride and lithium iodide using the hard-soft acid/base approach. Identify each Lewis acid and Lewis base, and categorize each as hard or soft.

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Charge Density – Hard Acids

Hard acids typically have a high charge density. They are often metal ions with a (higher) positive charge and small ionic size. Their d orbitals are often unavailable to engage in π bonding.

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Charge Density – Soft Acids

Soft acids typically have lower charge density (lower ionic charge and greater ionic size). Their d orbitals are available for π bonding. Soft acids are often 2nd and 3rd row transition metals with a +1 or +2 charge, and filled or nearly filled d orbitals.

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Acids

Hard Acids Borderline Soft Acids

H+, Li+, Na+, K+

Be2+, Mg2+, Ca2+

BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3

Al3+,Al(CH3)3,AlCl3,AlH3

Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,

Cu2+,Zn2+,Rh3+ Cd2+,Hg22+,

Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,

SO3 SO2 Pt4+

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Acids – Effect of Oxid’n #

Hard Acids Borderline Soft Acids

H+, Li+, Na+, K+

Be2+, Mg2+, Ca2+

BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3

Al3+,Al(CH3)3,AlCl3,AlH3

Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,

Cu2+,Zn2+,Rh3+ Cd2+,Hg22+,

Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,

SO3 SO2 Pt4+

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Bases

Hard Bases Borderline Soft Bases

F-, Cl- Br- H-, I-

H2O, OH-,O2- H2S, HS-, S2-

ROH, RO-, R2O, CH3CO2- RSH, RS-, R2S

NO3-, ClO4- NO2-, N3- , N2 SCN-, CN-,RNC, CO

CO32-,SO42-, PO43- SO32- S2O32-