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in the solid intermolecular O
···
I XBs that connect the anions into infinite
chains [126, 127].
Anions are usually better XB acceptors than neutral species [128] and the
more naked they are, the stronger their XB acceptor ability is. Most of the
halogen-bonded systems reported in the literature are formed by halide an-
ions. In this series, iodide examples are the most frequently recurring, and
fluoride anions are the least common. In solution, association of halide an-
ions with XB acceptors decreases in the order I
-
>
Br
-
>
Cl
-
>
F
-
[128]. This
scale contrasts with the scale based on percentage shortening of X
-
I-C
distances with respect to the sum of van der Waals radii. Clearly, the solva-
tion energies of the starting anions, and formed superanions, as well as the
strength of the formed XBs affect association processes and the relative rel-
evance of the two parameters is different in solution and in the solid. Also
polyhalide anions (e.g. I
3
-
[129], Br
3
-
[120], ICl
2
-
[111], ICl
4
-
[115]) form
XBs; usually they give longer, namely weaker, XBs than the corresponding
halides [112, 120, 128] and work as electron density donors at the two ends
of the ion. CN
-
···
[96-98, 130-132] and SCN
-
[133, 134] anions have been re-
ported to give XBs.
Typical nitrogen atoms (e.g. in amine and pyridine derivatives) give
stronger XBs than typical oxygen and sulfur atoms (e.g. in ethers, alco-
hols, thioethers) [67, 68, 135, 136]. When 1,2-diiodotetrafluoroethane and
N
-methylmorpholine interact [124], they both work as bidentate modules
and form infinite chains thanks to the formation of O
IXBs,
the two interactions corresponding to 0.82 and 0.80 times the respective sum
of van der Waals radii [137]. The relative effectiveness of oxygen and sulfur
in XB formation often depends on the nature of the XB donor, as the pair-
ings after the HSAB theory are favoured. A very rigorous and general scale
wherein XB acceptors, or donors, are listed according to their strength cannot
be filed as the relative effectiveness of a given set of XB acceptors (or donor)
may also depend on the used XB donor (or acceptor, respectively). Steric hin-
drance around the XB acceptor site decreases its effectiveness and may affect
the relative strength of two electron donor sites, as in the case for 2,2
-and
4,4
-BPY. Moreover, when subtle differences have to be compared, it may hap-
pen that scales based on thermodynamic and spectroscopic measurements
(or theoretical calculations) are not identical.
···
IandN
···
2.5
Directionality of XB
XB is a particularly directional interaction, more directional than HB. The
angle between the covalent and non-covalent bonds around the halogen in
D
X-Y is approximately 180
◦
[48]. As discussed above, the origin of this di-
rectionality is in the anisotropic distribution of electron density around the
halogen atom. Figure 5 shows the Cambridge Structure Database (CSD, ver-
···
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