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Likewise, the complexes of CO with the dihalogen molecules XY = ClF, [61]
Cl
2
, [39], BrCl [49], Br
2
[87] and ICl [94] are all linear in their equilibrium
geometries with atoms in the order OC
XY and with X as the more elec-
tropositive halogen atom when XY is a heteronuclear dihalogen. Thus, the
n-pair carried by C again defines the angular geometry in preference to the
π
···
pair. Complexes of the type N
2
···
HX, where X = F [150], Cl [151, 152]
and Br [153] are all linear, as are N
2
XY, where XY is ClF [71], BrCl [58]
and ICl [100]. Thus the complexes of N
2
also obey rule 3. The same pattern
emerges for the series of complexes formed by hydrogen cyanide with hy-
drogen halide molecules and with dihalogen molecules. Thus, each complex
has been shown to have a linear equilibrium geometry, with atoms in the
order HCN
···
···
HX, when X = F [154, 155], Cl, [156], Br [157] or I [158], or
HCN
XY, when XY = F
2
[32], ClF [64],Cl
2
[41], BrCl [53] or ICl [99]. Again,
when XY is a heteronuclear dihalogen, X is always the more electropositive
atom. Those members of the two series CH
3
CN
···
XY so
far investigated (namely HX = HF [159, 160] and HCl [161] and XY = F
2
[31]
and ClF [84]) indicate that the same conclusion appears to hold when methyl
cyanide is the electron donor. So, there is ample evidence that rule 3 holds for
both hydrogen- and halogen-bonded complexes.
Is there any evidence that this rule can be contravened? To answer this
question, the complexes of vinyl fluoride, furan and thiophene with HCl and
ClF will be considered. Vinyl fluoride, CH
2
CHF, is an example of a mixed n-
pair/
···
HX and CH
3
CN
···
-pair donor in which, unlike CO, HCN, CH
3
CN or CH
2
O, the pairs of
electrons (a
π
-pair shared between C
1
and C
2
and an n-pair on F) do not
have an atom in common. In addition, its complexes with HCl and ClF are
important in the context of linear/non-linear hydrogen and halogen bonds.
On the other hand, furan and thiophene are examples of mixed n-pair/
π
π
-pair
aromatic donors in which the n-pair can be withdrawn into the ring.
The geometries of complexes CH
2
CHF
ClF
[85], as determined from their ground-state spectroscopic constants, are dis-
played in Fig. 17. Each complex is effectively planar and we note that in each
case the F atom of vinyl fluoride acts as the electron donor. The simple elec-
tron density model showing the three n-pairs on F (see Fig. 17) leads to the
prediction of a value of
···
HCl [85, 162] and CH
2
CHF
···
115
◦
∼
for the angles C - F
···
HandC- F
···
Cl in
CH
2
CHF
=
123.7(1)
◦
and 125.7(3)
◦
, respectively, are very similar and reasonably close
to 115
◦
. This indicates that rule 3 is again obeyed. The angular deviations of
the F
···
HCl and CH
2
CHF
···
ClF, respectively. The observed values
φ
···
H - Cl nuclei and the F
···
Cl - Fnucleifromacollineararrangement
(defined as
in Fig. 17) are different, however. As is the case for several com-
plexes of C
S
symmetry discussed earlier, the halogen bond is strictly linear
[
θ
=0.6(1)
◦
]whilethehydrogenbonddeviatesby
= 18.3(1)
◦
θ
θ
from linear-
ity. The complexes vinyl fluoride
···
HF [163] and vinyl fluoride
···
HBr [164]
are isostructural with vinyl fluoride
···
HCl and exhibit similarly non-linear
hydrogen bonds.
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