Fused-ring metallabenzenes

• Fused ring metallabenzenes are a rapidly burgeoning class of organometallic compounds.
• Metallabenzenes with a diverse range of fused 3-, 5- and 6-membered rings are known.
• Experimental data suggest that fused-ring metallabenzenes have aromatic character.

Metallabenzenoids that contain a ring fused to the basic metallabenzene core represent a rapidly emerging new sub-category of metallabenzenes. There are a very large number of possible variations in the size, composition, degree of saturation and substitution patterns of the fused rings, as well as different isomer possibilities. At present compounds that incorporate fused 3-, 5- or 6-membered rings are all known and these are covered in separate sections of this review. The syntheses, spectroscopic properties, structures and reactions of the fused-ring metallabenzenes are discussed. Preliminary indications based on these data suggest that when π-electron delocalization can extend over the fused rings, the six-membered metallacyclic rings of the metallabenzenoids retain aromatic character, but at reduced levels compared to the parent metallabenzenes. Appropriate theoretical studies are needed to provide more reliable information about the bonding in fused-ring metallabenzenes.

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