An all-metal aromatic species Al

Benzene (CH)6, of course, is the most prototypal aromatic system. When one or more of the CH groups are replaced by other atom(s), a heterocyclic aromatic system is obtained. Well-known examples include pyridine N(CH)5 and pyrimidine N2(CH)4, while lesser known cases are phosphabenzene P(CH)5 and arsabenzene As(CH)s. There are also systems where the heteroatom is a heavy transition metal examples include L Os(CH)s and L Ir(CH)5. 

However, it is rare when all the atoms in an aromatic species are metals. One such system was synthesized in 2001 by A. I. Boldyrev andL.-S. Wang and their colleagues. Using a laser vaporization supersonic cluster source and a Cu/Al 

Before discussing the computational results, we will attempt to understand the bonding involved in the synthesized anions, using the elementary theories introduced in Chapter 3. First of all, it should be noted that the aromatic system referred to above is not the MA1J anion as a whole. Instead, structurally, the MAl anion should be considered as consisting of an M+ cation coordinated to a square-planar Al - unit and it is the dianion that has aromatic character. For Al -, there are 14 valence electrons and the resonance structures can be easily written  

In other words, for this square-planar dianion, there are four a bonds and one 7r bond, as well as two lone pairs. Hence each Al-Al linkage has a bond order of about Ea. Additionally, there are two it electrons in this planar ring system, satisfying the (An + 2)-rule for aromatic species. 

Now let us turn to the computational results. The structures of the square planar Al -, and square pyramidal LiAlJ, NaAlJ and CuAlJ are shown in Fig. 5.8.2. It is noted that the structure of CuAlJ has been optimized at the MP2/6-311+G(d) level, while the theoretical level for the remaining three species is CCSD(T)/6-311 +G(d). When these structures are examined, it is seen that the four-membered ring does not undergo significant structural change upon coordination to a M+ cation. This is especially true when the metal is lithium 

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