Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 49

number of coordinated cations (9 þ 0 þ 1 ¼ 6 þ 3 þ 1) is important for the

comparison.
g ¼ 1:1. The topological types pcu-b and nia play the key role in the cation

arrays of selenites MSeO 3 ; however, the dependence on the size of the M 2+ cation is
inverted compared to the oxoacid salts with triangular anions: large cations provide
the pcu-b motif with the T 33 coordination type of the anions (Fig. 3 ), while smaller
cations facilitate the formation of the NiAs (nia) array of vaterite type (not anti -
NiAs as for most structures considered in Sect. 3.1 ). Note that the NiAs-like
selenites (Fig. 4 ) are described as distorted perovskite-like structures [ 53 ]; these
descriptions, however, do not elucidate the relations to other MSeO 3 structure
types. The coordination types of the LO 3 2 anions (T 33 and T 6 ) correlate with
the size of the M 2+ cations. It seems reasonable to correlate the inversion with the
E-pair. However, the mechanism of this effect remains unexplained so far.
This effect gives rise to the appearance of other 6-coordinated underlying
nets (sev-6- C 2/ m and HIYXUJ) that are unique for this group of compounds. sev-
6- C 2/ m was generated ab initio [ 23 ] and has not been found in crystal structures so
far. The HIYXUJ underlying topology was revealed quite recently in the lead-
organic compound ( m 6 -3,4-dihydro-3-thioxo-1,2,4-triazin-5(2 H )-onate)Pb(II) [ 54 ]

(CSD Reference code HIYXUJ). Both topological types can be considered as
intermediate between pcu-b and nia because the topological indices of sev-6- C 2/
m and HIYXUJ lie between those of pcu-b and nia (Table 1 ). The same concerns
the pair of 5-coordinated underlying nets bnn and nia-5,5- P 2 1 / c ; the latter one can
be considered as a topological distortion of bnn that has a higher symmetry.
g ¼

2:1. The relations between the cation array Na 2 SinNa 2 SO 3 and the Ni 2 Al
structure type are discussed in detail in this volume [ 55 ] .
g ¼

2:3. The topological types of the underlying nets in M 2 (LO 3 ) 3 are prede-
termined by pyramidal (

1 -tetrahedral) form of the coordination figures of the
cations with the stereochemically active E-pair. The corundum motif (cor) is not
preferred in this case since the tetrahedral coordination of the 4-coordinated atom in
cor does not fit the

c

1 -tetrahedral form [ 56 ]. Therefore, two other 4,6-coordinated
nets, stp and NASTEH, appear; they contain square-planar coordination figures in
the most symmetrical embedding (Fig. 12 ). Both of them were revealed only in
metal-organic compounds [ 57 ] ; stp is quite common (16 examples), NASTEH was
found only in (ethane-1,2-diammonium)((

c

m 6 -ethylenediphosphonato) 2 Zn 3 )[ 58 ]

(CSD Reference code NASTEH).
g ¼

1:2. Two well-known topological types of AB 2 ionic compounds, rutile (rtl)
and pyrite (pyr), lie in the base of the M(SeO 3 ) 2 compounds (Figs. 11 and 13 ;

Fig. 12 Topological
resemblance between the
stp net ( P 6/ mmm ) and
Sc 2 (SeO 3 ) 3