Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 40

3 Underlying Nets and Cation Arrays in Inorganic
Oxoacid Salts

Inorganic oxoacid salts M y [LO 3 ] z or M y [XO 4 ] z with triangular, trigonal-pyramidal,
and tetrahedral anions is a good starting point to demonstrate the advantages of the
periodic-graph approach and the concept of underlying net since this class of ionic
compounds is numerous (more than 600 crystal structures according to Inorganic
Crystal Structure Database, release 2010/1), and the standard representation of
their crystal structures is conventional, clear, and simple. The structure units are
the cations M and oxoanions [LO 3 ] or [XO 4 ], i.e., oxygens are contracted to L or
X cations that represent the anions in the resulting underlying net. Thus, the
composition of the underlying net is M y L z or M y X z , i.e., it naturally describes
the cation array; the stoichiometric ratio g ¼

y:z is unambiguously determined by
charges of the ions. This quasi-binary representation assumes that ionic forces play
the most significant role in the formation of the periodic structure; the essentially
covalent bonding within the anions is considered local and their internal structure
is ignored. If the strength of bonds M-O is similar to that of bonds L(X)-O, one can
consider the crystal structure of an oxoacid salt as a whole; no simplification is
applied in this case. The relation to a binary compound can be found if the cations
M and L(X) are topologically equivalent; this is so-called grey isomorphism [ 15 ] .
Vice versa, the existence of “grey isomorphism” could be considered as an implicit
indication of similarity of bonding between cations and oxygens in an oxoacid salt;
however, this emerges rarely [ 15 - 17 ]. Note that in most cases the authors of original
structural investigations do not recognize the relations to binary compounds.
We have analyzed 569 crystal structures of oxoacid salts M y (LO 3 ) z (L

¼ B, C,

N, S, Se, Te, Cl, Br, I) and M y (XO 4 ) z (X Si, Ge, P, As, S, Se, Cl, Br, I), where the

underlying nets are 3-periodic. For all of them, the quasi-binary representation was
built and the resulting nets were classified with the TOPOS TTD collection. To
characterize the method of coordination of the oxoanion, we use the notation
X mbtk... [ 38 ] , where the letter X shows the ligand denticity (M, B, T, or K for
mono-, bi-, tri-, or tetradentate ligands, respectively), the integers mbtk

¼

are equal
to the numbers of the M atoms connected to one, two, three, or four atoms of the
oxoanion.

...

3.1 Oxoacid Salts with Triangular Anions

3.1.1 Carbonates

Carbonates are the most investigated group of the oxoacid salts with triangular
anions owing to their significance as minerals. They show rather strong relations
between chemical composition, synthesis conditions, and the topology of the
underlying nets (Table 2 ) .