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Table 3
Nucleophilicities
N
B
of Lewis bases B and electrophilicities
E
HX
of hydrogen
halides HX
B
N
2
CO
C
2
H
2
C
2
H
4
H
2
S
HCN
H
2
O
3
3
N
B
2.1
3.4
5.1
4.7
4.8
5.4
7.3
10.0
12.2
HX
HF
HCl
HBr
E
HX
10.0
5.0
4.2
bound according to this criterion. It is of interest to examine whether the
k
σ
of the B
XY series can be reproduced by assigning electrophilicities to the
various dihalogen molecules. The complexes B
···
F
2
are excluded from this
analysis because of insufficient
k
σ
values. We assume that
c
in Eq. 4 has the
same value (0.25 Nm
-1
) for both the B
···
···
HX and B
···
XY series and make the
arbitrary choice that
E
ICl
= 10.0. We then use
k
σ
ICl series [93-
102] in Eq. 4 to obtain the
N
B
values for N
2
,CO,C
2
H
2
,C
2
H
4
,HCN,H
2
O, H
2
S
and NH
3
showninTable4.
for the B
···
Table 4
Nucleophilicities
N
B
of Lewis bases B and electrophilicities
E
XY
of dihalogens XY
B
N
2
CO
C
2
H
2
C
2
H
4
HCN
H
2
O
H
2
S
3
3
N
B
2.1
3.2
4.8
5.6
5.8
6.3
6.3
6.6
12.2
XY
Cl
2
Br
2
BrCl
ClF
ICl
E
XY
4.1
6.0
7.4
8.2
10.0
ICl [102] to obtain
N
PH
3
because the
k
σ
for this
complex is anomalously high [20.7(1) N m
-1
]. Figure 19 shows the straight
line that necessarily results for the B
We d o n o t u s e
k
σ
for H
3
P
···
ICl series when
k
σ
is plotted against
N
B
. Also shown in Fig. 19 are the results for the other series B
···
···
XY, where
XY is Cl
2
, ClF, BrCl and Br
2
, when the
N
B
values determined from the se-
ries B
ICl are used with the appropriate
k
σ
values.Wenotethatforeach
series the points lie on a reasonable straight line, the slope of which is, ac-
cording to Eq. 4,
cE
XY
. Each line drawn results from a least-squares fit and
leads to the values
E
XY
for XY
=
Cl
2
,Br
2
, BrCl and ClF included in Table 4.
The value of
N
B
=6.3usedforPH
3
was that obtained by requiring that the
k
σ
for H
3
P
···
···
Cl
2
,H
3
P
···
Br
2
and H
3
P
···
BrCl fitted best onto the existing straight
lines for B
···
Cl
2
,B
···
Br
2
and B
···
BrCl, respectively. The anomalous nature of
the point for H
3
P
···
ICl (added subsequently) is then obvious. The
k
σ
values
for H
3
N
···
ClF and H
3
N
···
BrCl were not included in the least-squares fit for
···
···
the series B
BrCl, respectively, because they also appear to
be anomalously high. These high values are thought to arise from a non-
negligible charge transfer, as represented by a small contribution of the ionic
structure [H
3
NX]
+
ClF and B
Y
-
to the valence bond description of these complexes
(discussed in Sect. 5.2) [52, 63].
···
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