Bonding in Inorganic Aromatic Compounds

The Chemical Bond Chemical Bonding Across the Periodic Table, First Edition. 

In this chapter, we focus on aromaticity and antiaromaticity in inorganic compounds only. It means that the chemical species, which will be discussed, do not contain carbon atoms in their cyclic structures. There are a few review articles [17-25], which discuss different aspects of aromaticity and antiaromaticity in inorganic chemistry. Before we go further, we would like to outline the structure of this chapter. First, we describe criteria that are commonly used for probing aromaticity. Second, we consider inorganic aromatic molecules, which we loosely call conventional aromatic molecules. These molecules are simply isoelectronic species to one of the organic aromatic molecule. Third, we focus on what we loosely call unconventional aromatic molecules composed of main group atoms and transition-metal atoms. Finally, we conclude the chapter with a summary and short outlook. 

Unfortunately, chemists have not yet developed a single simple probe of aromaticity that can convince everybody that a particular system is aromatic. However, owing to the tremendous advancement of computational chemistry, one can use many criteria to assess aromaticity or antiaromaticity. When several different criteria show the presence of aroma ticity/antiaromaticy, the final conclusion is significantly more reliable. The list of criteria used by chemists at present are summarized in Table 14.1, which was initially proposed by Krygowski et al. [26]. 

These criteria have been formulated for Ji-aromatic and Jt-antiaromatic organic systems, but many of them are also applicable to a-aromatic and a-antiaromatic, as well as 5-aromatic and 8-antiaromatic systems. 

The criteria presented in Table 14.1 can be roughly broken into the following groups  

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The yellow to orange compounds 804(118207)2, 864(84 013)2, and 8e4(8b2Fii)2 have been prepared. Crystallographic studies on 804(118207)2 have shown that the cation 8c4 + has square-planar D h) geometry. The structure can be described by valence bond theory in terms of four resonance structures equivalent to (la), or by simple molecular orbital theory in which three of the four n molecular orbitals are filled. " The 8e4 + ions are examples of six-jr-electron systems, and they are thus examples of inorganic aromatic compounds (lb). 

Chemical Properties and Reactivity. LLDPE is a saturated branched hydrocarbon. The most reactive parts of LLDPE molecules are the tertiary CH bonds in branches and the double bonds at chain ends. Although LLDPE is nonreactive with both inorganic and organic acids, it can form sulfo-compounds in concentrated solutions of H2SO4 (>70%) at elevated temperatures and can also be nitrated with concentrated HNO. LLDPE is also stable in alkaline and salt solutions. At room temperature, LLDPE resins are not soluble in any known solvent (except for those fractions with the highest branching contents) at temperatures above 80—100°C, however, the resins can be dissolved in various aromatic, aUphatic, and halogenated hydrocarbons such as xylenes, tetralin, decalin, and chlorobenzenes. 

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

While the largest success of the unconventional aromaticity is certainly in the realm of bare metal clusters, the synthesis and characterization of the compounds containing [Pd4(p4-C9 H 9) (P4 -Cg H g) ] complex clearly demonstrated the importance of the delocalized 5-bonding and 5-aromaticity in inorganic, organometaUic, and coordination chemistry. 

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