Pearson hard and soft acid base theory

Lewis Acid/Base Theory

Acid: electron pair acceptor (electrophile)

Base: electron pair donor (nucleophile)

A/B reaction: complex formation where a new covalent bond is formed

Universal: any solvent, any phase, any chemical species 

Pearson�s Hard/Soft Acid/Base Theory

Hard acids or bases: high charge density, not polarizable

Soft acids or bases: low charge density, polarizable

Operating Principle: hard acids prefer to bind with hard bases and soft acids prefer to bind with soft bases

hard acid/base combinations tend to be more ionic

soft acid/base combinations tend to be more covalent

Nonpolarizable substances (hard species) have a large HOMO/LUMO gap so that the valence orbitals of the species are generally of very different energies; this means that orbital overlap is poor and transfer of electrons is more favorable than sharing; reverse for polarizable substances

HSAB can be used to qualitatively predict reactivity, especially metathesis reactions:

CuF(s)	+	HI(aq)		CuI(s)	+	HF(aq)
S H		H S		S S		H H

CaO(s)	+	2HBr(aq)		Ca2+(aq)	+	2Br-(aq)	+	H2O(l)
H H		H S		H		S		H H

Drago-Wayland

A + B A-B

–H = E_AE_B + C_AC_B

E_A, E_B : "electrostatic" interactions

C_A, C_B : "covalent" interactions

To get a large –H, need both E_A and E_B to be large or C_A and C_B to be large; molecules with similar binding preferences give more exothermic reactivity

	BF3	+	(CH3)3P		F3B-P(CH3)3
E	20.2		0.84
C	3.31		6.55	(kJ/mol units)

–H = (20.2)(0.84) + (3.31)(6.55) = 38.6 kJ/mol)

	BF3	+	(1,4-C4H8O2		F3B-O(C4H8)O
E	20.2		2.23
C	3.31		4.87	(kJ/mol units)

–H = (20.2)(2.23) + (3.31)(4.87) = 61.2 kJ/mol

Currently, Drago has added a third term to account for steric interactions