Chemistry Reference
In-Depth Information
Guided by these rules, we shall compare in most detail the observed angu-
lar geometries of pairs of complexes B
HCl for a wide range
of Lewis bases B, although we shall also refer to other B
···
ClF and B
···
···
XY and B
···
HX.
The reasons for choosing B
ClF as the series of halogen- and
hydrogen-bonded complexes for comparison are: (i) that these are by far
the most systematically studied of all the pairs of B
···
HCl and B
···
···
HX/B
···
XY series and
(ii) that deviations of the hydrogen bond atoms Z
···
H - Cl from collinearity
have been determined for a number of B
HCl of C
S
symmetry and are avail-
able for comparison with the corresponding quantity for B
···
ClF, similarly
determined. For each complex, the geometry was obtained by fitting the prin-
cipal moments of inertia of one or more isotopomers under the assumption of
unperturbed monomer geometries.
···
3.1
Angular Geometries of B···ClF and B···HCl in Which B is a n-Pair Donor
We begin by comparing pairs of B
HCl that test rule 1. We
shall show a diagram comparing the experimental geometries, drawn to scale,
of B
···
ClF and B
···
HCl for each B, together with a representation of B, also
to scale but with its n-pair(s) drawn in the form of an exaggerated electron
density distribution that is traditionally used among chemists. We shall then
employ a similar approach for various prototype
···
ClF and B
···
π
-electron donors to test
rule 2 and for mixed n- and
π
-donors appropriate to rule 3 in Sects. 3.2 and
3.3, respectively.
3.1.1
B Carries a Single n-Pair
TheprototypeLewisbasethatcarriesasinglen-pairandno
π
-pairs is am-
monia. The observed geometries of H
3
N
HCl [105]
are shown in Fig. 1, as is the n-pair model of NH
3
. Both complexes are
symmetric-top molecules belonging to the C
3
v
molecular point group and
clearly both obey rule 1. The geometries of the complexes H
3
N
···
ClF [63] and H
3
N
···
···
F
2
[30],
H
3
N
ICl [97] were
found also to be of C
3
v
symmetry and isomorphous with their H
3
N
···
Cl
2
[45], H
3
N
···
BrCl [52], H
3
N
···
Br
2
[86] and H
3
N
···
HX
counterparts (X = F
1
, Cl [105], Br [106] and I [107]). The halogen atom of
higher atomic number acts as the electron acceptor in complexes containing
a heteronuclear dihalogen molecule. The same conclusions have been reached
for the pairs H
3
P
···
XY, for XY
=
Cl
2
[40], BrCl [55], Br
2
[88] and ICl [102],
and their H-bonded analogues H
3
P
···
···
HX, where X = Cl [108], Br [109] and
I [110].
1
Howard BJ, Langridge-Smith PPR, unpublished observations
Search WWH ::
Custom Search