The concept of aromaticity was initially introduced in chemistry to account for the stability, reactivity, molecular structures, and other properties of many unsaturated organic compounds. Despite that, it has been extended to other species with mobile electrons including saturated systems, transition structures, and even inorganic molecules. In this review, we focus on the aromaticity of a particular family of organometallic compounds known as metallabenzenes, which are characterized by the formal replacement of a CH group in benzene by an isolobal transition metal fragment. In addition, aromaticity of related compounds such as heterometallabenzenes is considered as well. To this end, we shall describe herein the insight gained by the available experimental data as well as by the application of the state-of-the-art computational methods developed as descriptors for aromaticity together with a critical evaluation of their performance to quantitatively estimate the strength of aromaticity of these systems.

Aromaticity of metallabenzenes and related compounds

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Metallaaromatic Chemistry: History and Development.

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Recent Advances in Metallaaromatic Chemistry.

Organometallics of the Group 13 M−M Bond (M = Al, Ga, In) and the Concept of Metalloaromaticity

Transition-metal-containing metallabenzenes are an interesting class of compounds because they exhibit chemical properties of both organometallic compounds and aromatic organic compounds. The first stable transition-metal-containing metallabenzenes were reported in 1982 by Roper et al. Since then a number of stable metallabenzenes have been successfully isolated and characterized. Most of the wellcharacterized stable metallabenzenes are those with a transition metal atom from the third transition series, especially osmium, iridium, and platinum. In contrast, well-characterized metallabenzenes with a metal atom from the first or the second transition series are still very rare, although such species was predicted theoretically as early as 1979. Although coordinated metallabenzenes with metal atoms from the first and the second transition series and 1,3-dimetallabenzenes of Nb and Ta were isolated and well characterized, free metallabenzenes with a metal atom from the first or the second transition series have been isolated rarely. The ruthenabenzene [CpRu{C(Ph)CHCHC(Ph)C(OEt)}(CO)] (Cp= cyclopentadienyl), and related ruthenaphenoxide and ruthenaphenanthrene oxide reported by Jones and co-workers represent rare examples (and the only reported examples, to the best our knowledge) of spectroscopically characterized metallabenzenes with a metal from the second transition series. However, these species are thermally unstable at ambient temperature. Other metallabenzenes such as ferrobenzene, ruthenaphenanthrene, ruthenaphenol, and chromobenzene have been proposed as reactive intermediates, but no direct observation of such species have been made. Herein we report the isolation and characterization of thermally stable ruthenabenzenes. We recently reported a convenient method for the preparation of osmabenzenes starting from the reactions of [OsCl2(PPh3)3] with HC CCH(OH)C CH. We found that the method can also be used to prepare ruthenabenzenes. Treatment of [RuCl2(PPh3)3] (1) [17] with HC CCH(OH)C CH, PPh3, and Bu4NCl in THF produced cationic ruthenabenzene 2, which was isolated as a green solid in 55% yield (Scheme 1). Interestingly, 2 can even be obtained from the one-pot reaction of RuCl3, PPh3, and HC CCH(OH)C CH in CHCl3, although the yield is lower.

Synthesis and characterization of stable ruthenabenzenes

Synthesis, Spectroscopy, and Structure of a Family of Iridabenzenes Generated by the Reaction of Vaska-Type Complexes with a Nucleophilic 3-Vinyl-1-cyclopropene†

Metallabenzenes and Valence Isomers. 3. Unexpected Rearrangement of Two Regioisomeric Iridabenzenes to an (η5-Cyclopentadienyl)iridium(I) Complex

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Synthesis, Spectroscopy, and Structure of a Family of Iridabenzenes Generated by the Reaction of Vaska-Type Complexes with a Nucleophilic 3-Vinyl-1-cyclopropene†

Metallabenzenes and Valence Isomers. 3. Unexpected Rearrangement of Two Regioisomeric Iridabenzenes to an (η5-Cyclopentadienyl)iridium(I) Complex