HSAB Flashcards
What is a Lewis base
An electron donor
eg. OH- , F- , H2O, ROH, NH3, SO42- , H- , CO, PR3, C6H6.
What is a Lewis acid
An electron acceptor
eg. H+; K+ , Mg2+, Fe3+; BF3, CO2, SO3, RMgX, AlCl3, Br2
How are the interactions of Lewis acids and Lewis bases showed
- In two classes of substance
- The two classes identified by their trends in the
stabilities of complexes that they form
Explain R.G. Pearson
- Suggested the terms hard and soft for the two classes, that were formerly simply known as “class a” and “class b”
Explain Pearson’s principle
- Hard acids prefer to bind to hard bases
- Soft acids prefer to bind to soft bases
Explain Hard acids and bases fully
- Small, not easily polarizable
- Ionic bonding and interactions predominant
- Ti4+, Cr3+, Fe3+, Co3+ and H+
(Metal ion: Alkali metals, alkaline earth metals,
lighter transition metals in higher oxidation states
Explain Soft acids and bases fully
- Large, polarizable
- Bonding is predominantly covalent in character
- Cu+, Ag+, Hg+, Hg2+,Pd2+, Pt 2+
(Metal ion: Heavier transition metals,
transition metals in lower oxidation states)
Explain Hard base: Ligands
- Incl. halides and oxo-anions)
- Ionic bonding and interactions are predominant
eg. NH3, F−, OH−, H2O
Explain Soft base Ligands
- Like Phosphines (PR3), iodide (I−), CO, CN−
- Bonding is predominantly covalent in character
The base B may be classified as hard or soft by the behavior of the a certain equilibrium
Explain
- It is possible to classify any given acid or base as hard or soft by its preference for hard or soft reactants
- BH+ + CH3Hg+ ——-> CH3Hg B+ + H+
- In this competition between a hard acid (H+) and a soft acid (CH3Hg+), a hard base will cause the reaction to go to the left: then the hard acid (H+) and hard base are together (BH+)
- A soft base will cause the reaction to proceed to the right: then the soft acid (CH3Hg+) and soft base are together (CH3HgB+) as product
How are hard and soft bases useful
- Useful in predicting the stability of complexes, systematize complex formation
- Cr3+ (hard acid) complexes often associated with H2O, OH− and O2−
(hard bases) - Pt2+ (soft acid) complexes often associated with PR3 (phosphines)
and CO ligands (soft bases)
What is to be noted with regards to hardness and softness
- Hardness and softness refer to special stability of hard-hard and soft-soft interactions and should be carefully
distinguished from inherent acid or base strength. - OH− and F−, are both hard bases but basicity of OH− is about 1013 times that of F−
Explain SCN− as a base
- A base that comprises both the hard N atom and the soft S atom
- SCN− binds to the hard acid Cr3+ through N (hard): Cr−NCS, but
- SCN− binds to the soft acid Pt2+ through S (soft): Pt−SCN
- N is hard, but P is soft (N smaller than P)
- O is hard, but S is soft (O smaller than S)
Explain soft acids for halogens
- Soft acids bond in the order: I− > Br− > Cl− > F− ( up period it incr )
- Equilibrium constant Kf increase from F− to I−
- Hg2+ complex (soft) with I− (soft) more stable than with F−
Explain hard acids for halogens
- Hard acids bond in the order: F− > Cl− > Br− > I−
- Equilibrium constant Kf increase from I− to F−
- Al3+ complex (hard) with F− (hard) more stable than with I−
