Page 178 - Mechanism and Theory in Organic Chemistry

Lewis Acids and Bases 167

Table 3.20 PEARSON'S CLASSIFICATION LEWIS ACIDS AND BASES
OF
Hard Borderline Soft
Acids '
H+, Li+, Na+, K+ Fez+, Co2+, Ni2+ Cu+, Ag +, Hg+
Be2+, Mg2+, Ca2+, Sr2+, Mn2+ Cu2+, Zn2+ Hg2 +
A13+, Cr3+, Co3+, Fe3+ Pb2 +, Sn2 + BH,, RS+, I+
BF3, B(OR), B(CH313, SO2 Br+, HO+, RO+
A1(CH3),, AlCl,, AlH, NO+, R,C+ 12, Br2
CeH5+ Trinitrobenzenc, etc.
RPO, + , ROPO, +
RS02+, ROS02+, SO3 Chloranil, quinones, etc.
RCO+, CO,, NC+ Tetracyanoethylene, etc.
HX (hydrogen-bonding CH,, carbenes
molecules)
Bases
H,O, OH-, F- C13~5~~2, C5H5N R2S, RSH, RS-
CH,COO -, PO,, -, S042- N3-, Br-, NO2- I-, SCN-, Sz03-
C1-, CO3'-, C1O4-, Nos- SOa2- R3P, (RO)3P
ROH, RO-, R20 Nz CN-, RNC, CO
NH,, RNH, C2H4, CeH6
H-, R-
-- -- -
SOURCE: R. G. Pearson, Survey of Progress in Chemistry, 5, 1 (1969). Reproduced by permission of
Academic Press and R. G. Pearson.
least a qualitative way with the hard-soft principle.lzZ Pearson has emphasized
that the HSAB principle is meant to be used only qualitatively, as a way of
systematizing experimental results, and we should heed this warning.lZ3

Applications of the HSAB principle In considering Brernsted acidities,
we have already met some equilibria to which we can apply the hard-soft ideas.
In Table 3.3 we noted that within a given column of the periodic table, the
hydrides become more acidic as one moves down. The negative ions in the lower
rows are softer bases than the corresponding ones in the upper rows (HzP- is
softer than HzN-; HS- is softer than HO-), and the softer bases bond less
strongly to the hard proton. The limited gas-phase data (Table 3.8) suggest that
the same order applies: RSH is a stronger gas-phase acid than ROH. Similarly,
the hard neutral nitrogen is more basic toward hard proton than is softer neutral
phosphorus, both in the liquid (Table 3.12) and in the gas (Table 3.18). Neutral
oxygen and sulfur bases, however, appear to be of comparable basicity toward
the proton in the gas phase (Table 3.18). We shall find other applications for the
HSAB principle in later chapters.
The theoretical basis for the hard-soft principle It is worthwhile at
this point to discuss briefly some of the theoretical concepts behind the hard-soft

(a) R. G. Pearson, H. Sobel, and J. Songstad, J. Amer. Chem. Soc., 90, 319 (1968); (b) C. D.
Ritchie, Accts. Chem. Res., 5, 348 (1972).
Pearson's hard-soft scheme has been criticized: (a) R. J. P. Williams and J. D. Hale, Structure and
Bonding, Vol. 1, Springer-Verlag, Berlin, 1966, p. 249; (b) R. S. Drago and R. A. Kabler, Inorg. Ch.,
11, 3144 ((1972); (c) R. G. Pearson, Inorg. Chem., 11, 3146 (1972); (d) R. S. Drago, Inorg. Chem., 12,
2211 (1973).