8.4 Ligand Close-Packing (LCP) Model
217
Table 8.6 Bond lengths (pm), bond angles (degrees), F…F distances (pm), and r L (F) ligand radii
(pm) in some molecules containing the CF n group Source of structural data: MOGADOC database,
Vogt et al. (2015)
X
χ(X) a
r e (X–F)
∠ e (FXF)
d(F…F)
r L (F)
SiF 4
Si
1.9
155.2
109.5
253.5
126.8
BF 3
B
2.0
130.7
120.0
226.4
113.2
AsF 3
As
2.2
170.4
95.9
253.1
126.5
PF 3
P
2.2
156.1
97.6
234.9
117.4
CF 4
C
2.6
131.5
109.5
214.8
107.4
SF 2
S
2.6
158.7
98.0
239.7
119.8
NF 3
N
3.0
136.8
101.9
212.3
106.2
F 2 0
O
3.4
140.5
103.1
220.1
110.0
a Electronegativity of atom X (Pauling scale)
reach 180° in OX 2 E 2 molecules and 120° in AX 3 E molecules. For instance,
contrary to H 2 O, Li 2 O is linear.
3. Bond lengths for an A–X bond increases with the coordination number because
of the ligand–ligand repulsions. For instance, r(F) is 124.4 pm in the planar
equilateral molecule CF 3
+ and 131.9 pm in tetrahedral CF 4 but, as expected, the
interligand distance d(F…F) is nearly the same in both molecules.
4. When a ligand is replaced by another ligand of comparable size but with a shorter
bond, it pushes away the other ligands with longer bonds increasing the angle
between short and long bonds. For instance, the ∠(CFC) angle is 120° in CF 3
+
and the r(CF) length is 123.5 pm, whereas in O=CF 2 , the ∠(OCF) angle is 126.1°
and r(CO) = 117.0 pm, but the interligand distance d(F…F) is nearly the same
in both molecules, 214 pm and 212 pm, respectively.
5. The distance between two different ligands X and Y is given approximately by
the sum of the ligand radii of X and Y. For instance, r L (F) = 107.5(11) pm and
r L (Cl) = 144.8(9) pm, give the mean value d(F…Cl) = 252 pm which may be
compared with the equilibrium values given in Table 8.7.
Table 8.7 Interligand F…Cl distance (pm) (Vogt et al. 2014)
d(F…Cl)
r(CF)
r(CCl)
∠(FCCl)
OCClF
249.7
132.3
172.1
109.6
CCl 2 F 2
252.6
132.9
175.2
109.35
CClF 3
253.1
132.1
175.0
110.27
CHClF 2
254.0
133.6
175.6
109.62
CHCl 2 F
254.1
134.4
175.7
109.34
CH 2 ClF
257.2
135.9
176.4
110.02
Accurate equilibrium structures of fluoro- and chloroderivatives of methane; Vogt et al. 2014;
reprinted by permission of the publisher Taylor & Francis Ltd., http://www.tandfonline.com
217
Table 8.6 Bond lengths (pm), bond angles (degrees), F…F distances (pm), and r L (F) ligand radii
(pm) in some molecules containing the CF n group Source of structural data: MOGADOC database,
Vogt et al. (2015)
X
χ(X) a
r e (X–F)
∠ e (FXF)
d(F…F)
r L (F)
SiF 4
Si
1.9
155.2
109.5
253.5
126.8
BF 3
B
2.0
130.7
120.0
226.4
113.2
AsF 3
As
2.2
170.4
95.9
253.1
126.5
PF 3
P
2.2
156.1
97.6
234.9
117.4
CF 4
C
2.6
131.5
109.5
214.8
107.4
SF 2
S
2.6
158.7
98.0
239.7
119.8
NF 3
N
3.0
136.8
101.9
212.3
106.2
F 2 0
O
3.4
140.5
103.1
220.1
110.0
a Electronegativity of atom X (Pauling scale)
reach 180° in OX 2 E 2 molecules and 120° in AX 3 E molecules. For instance,
contrary to H 2 O, Li 2 O is linear.
3. Bond lengths for an A–X bond increases with the coordination number because
of the ligand–ligand repulsions. For instance, r(F) is 124.4 pm in the planar
equilateral molecule CF 3
+ and 131.9 pm in tetrahedral CF 4 but, as expected, the
interligand distance d(F…F) is nearly the same in both molecules.
4. When a ligand is replaced by another ligand of comparable size but with a shorter
bond, it pushes away the other ligands with longer bonds increasing the angle
between short and long bonds. For instance, the ∠(CFC) angle is 120° in CF 3
+
and the r(CF) length is 123.5 pm, whereas in O=CF 2 , the ∠(OCF) angle is 126.1°
and r(CO) = 117.0 pm, but the interligand distance d(F…F) is nearly the same
in both molecules, 214 pm and 212 pm, respectively.
5. The distance between two different ligands X and Y is given approximately by
the sum of the ligand radii of X and Y. For instance, r L (F) = 107.5(11) pm and
r L (Cl) = 144.8(9) pm, give the mean value d(F…Cl) = 252 pm which may be
compared with the equilibrium values given in Table 8.7.
Table 8.7 Interligand F…Cl distance (pm) (Vogt et al. 2014)
d(F…Cl)
r(CF)
r(CCl)
∠(FCCl)
OCClF
249.7
132.3
172.1
109.6
CCl 2 F 2
252.6
132.9
175.2
109.35
CClF 3
253.1
132.1
175.0
110.27
CHClF 2
254.0
133.6
175.6
109.62
CHCl 2 F
254.1
134.4
175.7
109.34
CH 2 ClF
257.2
135.9
176.4
110.02
Accurate equilibrium structures of fluoro- and chloroderivatives of methane; Vogt et al. 2014;
reprinted by permission of the publisher Taylor & Francis Ltd., http://www.tandfonline.com