Chemistry Reference
In-Depth Information
Fig. 10 The FeB net (B-B
contacts are ignored) and
some related structures of
oxoacid salts.
Anti
-FeB:
KNO
3
-IV; normal FeB:
BaSO
4
(barite) and
isostructural MSO
4
, BaSeO
4
,
MClO
4
, and MBrO
4
Fig. 11 The pyrite-like net
(
Pa
3, S-S contacts are
ignored) and some related
structures of oxoacid salts: Ca
(NO
3
)
2
and isostructural M
(NO
3
)
2
(
Pa
3);
a
-Sn(SeO
3
)
2
topologically less dense underlying net (pcu-b, topological density TD
10
¼
1,561)
than
a
-KNO
3
(nia,TD
10
¼
1,748).
1:2. Almost all nitrates M(NO
3
)
2
investigated by X-rays have the same
pyrite-like (pyr) 3,6-coordinated underlying net (Fig.
11
); however, the space-
group symmetries vary from orthorhombic (
Pca
2
1
,M
g
¼
¼
Cd) to rhombohedral
(R3, M
Co) and cubic (
Pa
3, M
Ca, Sr, Ba, Mn, Ni); the highest symmetry
coincides with the maximal-symmetry embedding of the pyr net. The coordination
numbers of the M atoms also change from 6 (M
¼
¼
¼
Mn, Co, Ni) to 7 (M
¼
Cd) and
12 (M
Ca, Sr, Ba), which results in the corresponding change in coordination
type of the nitrate groups: from T
3
¼
6toT
03
for CN
¼
for CN
¼
12. A special
(square) coordination of copper atoms in
-Cu(NO
3
)
2
gives rise to the moganite
(mog) motif that is nontypical for cation arrays.
b
3.2 Oxoacid Salts with Trigonal-Pyramidal Anions
This group of ionic compounds is characterized by an asymmetric environment of
anions owing to stereochemically active lone electron pair (E-pair). This distortion,
being rather strong, often does not allow one to recognize the topological motif of
the underlying net just with a visual analysis of the crystal structure. Indeed, the
number of correspondences to binary compounds is less compared to the oxoacid
