Inorganic Chemistry
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Inorganic Chemistry

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for purely ionic compounds because cations are closer 
together than in the rocksalt structure, is often found with transition metals in combination with less 
electronegative nonmetals such as S, P and As. The compounds formed are of low ionic character 
and frequently show metallic conduction. The close contacts between metal atoms facilitate direct 
bonding interaction. 
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Section D\u2014Structure and bonding in solids 
Homoelement bonding 
Bonding between atoms of the same kind may often be present when a binary compound shows an 
apparently anomalous stoichiometry. For example, the solids with empirical formulae NaO, KO2, 
LiS, CaC2 and NaN3 contain the ions and respectively. Combination of an 
electropositive metal with a p-block element of intermediate electronegativity gives so-called Zintl 
compounds. Some contain discrete polyatomic units such as Ge4 tetrahedra in KGe; in others there 
are continuous bonded networks such as Si chains in CaSi, or layers in CaSi2. Often these structures 
can be understood by isoelectronic analogy with the nonmetallic elements (see Topics C1 and D2): 
thus Ge4
4\u2212 (in KGe) has the same valence electron count as P4; Si
2\u2212 (in CaSi) is similarly 
isoelectronic to S, and Si\u2212 (in CaSi2) to P. Although this analogy is useful the ionic formulation may 
be misleading, as the solids are often metallic in appearance and are semiconductors. 
The term metal-metal bonding is used when such homoelement bonding involves the more 
electropositive element of a binary pair. Again, it may sometimes be present when an unusual 
oxidation state is found. For example, HgCl contains molecular Hg2Cl2 units with Hg-Hg bonds, and 
GaS also has Ga-Ga bonds (see Topics G4 and G5). Metal-rich compounds are formed by early 
Key Notes 
Binary solids with bonds between atoms of the same type include compounds with ions 
such as Zintl compounds formed between electropositive metals and p-block 
elements of period 3 and below, and compounds with metal-metal bonding often formed 
by 4d and 5d transition metals. 
Ternary structures Some ternary oxides and halides may have discrete complex ions such as others 
have structures with no such discrete ions. Silicates show a range of intermediate 
possibilities. The compound formula alone does not indicate the structure type. 
Microporous solids Zeolites are solids with aluminosilicate frameworks having pores and channels. When 
these are occupied by hydrated ions the compounds are used as ion exchangers; when 
the pores are empty they have useful catalytic properties. 
Intercalation and 
Intercalation compounds are formed from layered structures with additional atoms or 
molecules between the layers, insertion compounds when atoms enter a three-
dimensional framework. Many of these compounds are nonstoichiometric. 
Related topics Inorganic reactions and synthesis (B6) 
Binary compounds: simple structures (D3) 
Oxygen (F7) 
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transition metals, with formulae such as Sc2Cl3 and ZrCl, and structures showing extensive metal-
metal bonding. They are especially common with elements of the 4d and 5d series and sometimes 
may not be suspected from the stoichiometry. An example is MoCl2, which contains the cluster 
4+ with a metal-metal bonded Mo octahedron (see Topic H5). Metal-metal bonding often 
gives rise to anomalous magnetic or other properties, but the surest criterion is a structural one, with 
metal-metal distances comparable with or shorter than those found in the metallic element. 
Ternary structures 
Ternary structures are ones with three elements present, examples being CaCO3 and CaTiO3. 
Oxides are the commonest examples of such structures and exemplify some of the important 
principles (see Topic F7). Two fundamentally different structural features are possible, as follows. 
This division is not absolute, however, and the varied structures of silicates provide examples of 
intermediate cases. ZrSiO4 (zircon) has discrete ions but silicates such as CaSiO3 do not 
contain individual units but are formed from tetrahedral SiO4 groups sharing corners to make 
rings or infinite chains (see Topic D3, Fig. 3). Further sharing of corners can make two- and three-
dimensional networks. The different structures of carbonates and silicates reflect some typical and 
very important differences in bonding preference between periods 2 and 3 in the p block (see Topics 
F1 and F4). 
Complex oxides are normally found when a nonmetal is present, with oxoanions such as nitrate 
 carbonate phosphate or sulfate but are also sometimes formed by metals 
in high oxidation states (e.g. permanganate in KMnO4). When a compound contains two 
metallic elements the mixed oxide form is more normal, but it is important to note that the compound 
formula itself provides very little guide to the structure (compare CaCO3 and CaSiO3 above). A 
similar structural variety is found with complex halides. For example, the K2NiF4 structure is based 
on layers of corner-sharing NiF6 octahedra with no discrete complex ions, whereas K2PtCl4 contains 
individual square planar ions [PtCl4]
2\u2212. These differences reflect the bonding preferences of NiII and 
PtII (see Topics H4 and H5). 
\u2022 Complex oxides are compounds containing complex ions, which appear as discrete structural 
units. For example, calcium carbonate has a structure based on rocksalt with the different sites 
occupied by Ca2+ and ions. 
\u2022 Mixed oxides are exemplified by CaTiO3, which, although often called \u2018calcium titanate\u2019, does 
not have discrete titanate ions. The perovskite structure (Fig. 1) shows a corner-sharing 
network of TiO6 octahedra (essentially the ReO3 structure; see Topic D3, Figs 1 and 3) with 
Ca2+ occupying the large central site coordinated by 12 oxygen ions. 
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Fig. 1. Unit cell of the perovskite structure of CaTiO3. 
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Microporous solids 
Zeolites are aluminosilicate solids based on a framework of corner-sharing SiO4 and AlO4 
tetrahedra. These frameworks contain pores and channels of molecular dimensions, which in natural 
minerals (or after laboratory synthesis, usually by hydrothermal methods, see Topic B6) contain 
species such as water and hydrated ions. Removal of these species (e.g. by careful heating under 
vacuum) leads to microporous materials with empty channels and pores. It is possible to make 
synthetic zeolites of composition SiO2 with no aluminum, but when Al
III is present the framework 
formula is [AlxSi1\u2212xO2]
x\u2212 and the charge must be compensated by extra-framework cations. In as-
prepared zeolites these may be alkali cations, or organic amines, but when the pore materials 
are removed they are replaced by H+, which forms strong Brønsted acid sites within the pores. 
The structure of the zeolite faujasite is shown in Figure 2. In this conventional representation the 
framework structure is shown without depicting atoms directly. Each line represents an Si\u2014O\u2014Si 
or Si\u2014O\u2014Al connection. Four lines meet at tetrahedral vertices representing the positions of the 
four-coordinate Si or Al atoms. Space-filling models of this zeolite show that the pores can 
accommodate molecules up to about 750 pm in diameter. 
In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening 
by replacement