Page 28 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
8
To a significant extent, the high nucleophilicity of “naked” fluoride ions may be attributed
to the strength of the C–F bond (more on which in Section 1.6). Because the S 2 transition
N
state involves bond formation between the incoming nucleophile and carbon, the strength
of that bond is a key determinant of nucleophilicity.
Last but not least, steric effects are yet another key determinant of nucleophilicity. We
will discuss steric effects to some extent in Section 1.7.

1.3 HARD AND SOFT ACIDS AND BASES: THE HSAB PRINCIPLE

Several of the factors affecting nucleophilicity may be nicely rolled together into the
concept of hard and soft Lewis acids and bases—HSAB, for short. The HSAB concept
was introduced by Ralph Pearson over 50 years ago and was subsequently put on a firmer
theoretical foundation by Pearson and Parr, among others. Hard acids and bases are
relatively unpolarizable and have relatively high surface charge density, positive or
negative; soft acids and bases are relatively polarizable and have low surface charge
density, positive or negative. Of course, there are many borderline cases. High surface
charge density (hardness) typically results from a high formal charge (FC), positive or
negative, and small atomic/ionic size, and the opposite is true for low surface charge density
(softness). Examples of hard, borderline, and soft acids and bases are shown in Table 1.3.
The utility of the hardness/softness concept derives from the HSAB principle, which
states that soft bases react faster and form stronger bonds with soft acids, and hard bases
react faster and form stronger bonds with hard acids. A vast amount of chemistry can be
rationalized with this principle.
The HSAB concept greatly facilitates our appreciation of nucleophilicity: softer bases
often make better nucleophiles. Phosphines, for example, are typically better nucleophiles
than the analogous, harder amines, and sulfur compounds are better nucleophiles than their
oxygen analogs.
In this book, some of the best illustrations of the HSAB principle will be provided by
the so-called ligand exchange or metathesis reactions, which are discussed in more detail in
Section 1.19. The principle helps us in deciding whether a metathesis reaction will proceed
in a given direction or not:
AB + CD → AC + BD (1.3)

TABLE 1.3 Qualitative Listing of Hard, Intermediate, and Soft Acids and Bases a
Acids Bases
+
+
+
Hard H , H–X, Li ,Na ,R SiX NH ,RNH 2
3
3
2+
2+
−
−
Mg ,Ca , AlX , SnCl , TiCl 4 H O,HO ,ROH, RO ,RCO 2 −
3
2
4
−
−
Cl ,F ,NO 3 −
−
−
Intermediate CuX , ZnX , SnX , GaX , Br ,NNN (azide), ArNH 2
3
2
2
2
+
R C ,R B pyridine
3 3
−
−
−
Soft RCH X, RSX, RSeX, I ,Br , BrF , RSH, RS ,R S,RSe ,I ,R P,
2 2 2 3 2 3
−
+
CuX, Ag , Pd(X/R) , Pt(X/R) , NC , CO, RCH=CHR,
2 2
Hg(X/R) , zero-valent metals benzene
2
a
Where warranted, the atom of interest is indicated in bold.