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the electrophilicity index, etc., using which we can rationalize the varia-
tion of chemical properties of the elements. The parameters are known
as fundamental parameters. Based on a fundamental atomic parameter,
polarizability, Pearson [9], in an attempt to systematize and rationalize the
intrinsic property of atoms, ions and molecule, classifi ed them into three
classes: —hard, soft, and borderline. It should be noted that by polarizabil-
ity, Pearson [9] meant the ease of deforming the valence electron cloud of
a chemical species. Division into these two categories is not absolute and
intermediate cases occur, which was termed as “borderline.”
(1) Hard acid —those that bind strongly to bases which bind strongly
to the proton, that is, basic in the usual sense; the acceptor atom is of high
positive charge, small size, and has no easily polarized outer electrons.
(2) Soft acid —those that bind strongly to highly polarizable or un-
saturated bases, which often have negligible proton basicity. The acceptor
atom is of low positive charge, large size and has polarizable outer elec-
trons.
(3) Soft base —the donor atom is of low electronegativity, easily oxi-
dized, highly polarizable, and with low-lying empty orbitals.
Some hard-soft and borderline Lewis acids and bases are presented in
Tables (1.1) and (1.2), respectively.
(4) Hard base —the donor atom is of high electronegativity, hard to
oxidize, of low polarizability, and with only high energy empty orbitals.
TABLE 1.1
Classification of Lewis acids
Hard acid or class (a) acid
Borderline
Soft acid or class (b) acid
H + ,Li + ,Na + ,K + ,Be 2+ ,Mg 2+ ,
Ca 2+ ,Sr 2+ ,Sn 2+ ,Al 3+ ,Se 3+ ,
Ga 3+ ,In 3+ ,La 3+ ,Cr 3+ ,Co 3+ ,
Fe 3+ ,As 3+ ,Ir 3+ ,Si 4+ ,Ti 4+ ,Zr 4+ ,
Th 4+ ,Pu 4+ ,VO 2+ ,UO 2 2+ ,(
CH 3 ) 2 Sn 2+ , BeMe 2 ,BF 3 ,
BCl 3 ,B(OR) 3 ,Al(CH 3 ) 3 , Ga
(CH 3 ) 3 ,In(CH 3 ) 3 RPO 2 + ,RSO
2
Fe 2+ ,Co 2+ ,Ni 2+ ,Cu 2+ ,Zn 2+ ,
Pb 2+ ,B(CH 3 ) 3 , SO 2 ,NO +
Cu + ,Ag + ,Au + ,Tl + ,Hg + ,Cs + ,
Pd 2+ ,Cd 2+ ,Pt 2+ ,Hg 2+ ,CH 3 Hg +
Tl 3+ ,Tl(CH 3 ) 3 , RH 3 , RS + ,
RSe + , RTe + , I + ,Br + ,HO + ,
RO + ,I 2 ,Br 2 ,INC, etc,
C 6 H 3 (NO 2 ) 3 ,
2,3,5,6-Tetra chloro-p-
benzoquinone(chloranil),
Tetracyanoethylene(C 6 N 4 )
O,Cl,Br,I,R 3 C, neutral metal
ions(M 0 ), Bulk metals
+ ,ROSO 2 + , SO 3 ,I 7+ ,I 5+ ,Cl 7+ ,
R 3 C + ,RCO,CO 2 ,NC + ,HX(H-
bonding molecules)
 
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