Hard-Soft Acid-Base Theory

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NH3, RNH2 C6H5NH2, pyr R3P, C6H6

Slide 28

Bases – effect of Oxid’n #

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-

NH3, RNH2 C6H5NH2, pyr R3P, C6H6

Slide 29

Effect of Linkage Site

SCN- vs. NCS-

The nitrogen tends to coordinate with harder acids such as Si, whereas the sulfur tends to coordinate with softer acids such as Pt2+.

Slide 30

Effect of Oxidation Number

Cu2+/Cu+ on acid hardness

SO3/SO2 on acid hardness

NO3-/NO2- on base hardness

SO42-/SO32- on base hardness

Slide 31

Acid or Base Strength

It is important to realize that hard/soft considerations have nothing to do with acid or base strength. An acid or a base may be hard or soft and also be either weak or strong.

In a competition reaction between two bases for the same acid, you must consider both the relative strength of the bases, and the hard/soft nature of each base and the acid.

Slide 32

Acid or Base Strength

Consider the reaction between ZnO and LiC4H9.

ZnO + 2 LiC4H9↔ Zn(C4H9)2 + Li2O

Zinc ion is a strong Lewis acid, and oxide ion is a strong Lewis base.

Slide 33

Acid or Base Strength

Consider the reaction between ZnO and LiC4H9.

ZnO + 2 LiC4H9↔ Zn(C4H9)2 + Li2O

Zinc ion is a strong Lewis acid, and oxide ion is a strong Lewis base. However, the reaction proceeds to the right (K>1), because hard/soft considerations override acid-base strength considerations.

soft -hard

hard -soft

soft -soft

hard -hard

Slide 34

The Nature of the Adduct

Hard acid/hard base adducts tend to have more ionic character in their bonding. These are generally more favored energetically.

Soft acid/soft base adducts are more covalent in nature.

Slide 35

Other Considerations

As the adduct forms, there is usually a change in geometry around the Lewis acid site.

BX3 + N(CH3)3  X3B-NMe3

The stability of the adduct is:

BBr3 > BCl3> BF3

This order seems opposite of what would be expected based on halogen size or electronegativity.