Metallabenzenes.

Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring pi-bonds. However, it is also apparent that metallabenzenes are much more prone to isomerization reactions than are conventional arenes. This appears to be particularly true of first-row and second-row metallabenzenes, where the metal-carbon bond strengths are weaker. In these systems, carbene migratory insertion often leads to cyclopentadienyl-metal products. pi-Coordination of metallabenzenes to other metal centers generally stabilizes the metallabenzene moieties while maintaining their aromatic character. Among metallabenzenes coordinated in this way, there are representatives from all three transition-metal rows (Fe, Ni, Mo, Ru, and Ir). The metal atom in pi-coordinated metallabenzenes is displaced out of the ring and away from the complexing metal center. The reason for this displacement in most cases appears to be steric repulsion between ligands on the two metal centers. However, other subtle effects may contribute. For example, metal displacement leads to more favorable internal angles at the alpha-carbons and a better orientation of C alpha p orbitals toward the complexing metal center. While no dominant synthetic strategy for constructing metallabenzenes has emerged, cyclization reactions involving metal-thiocarbonyl, metal-alkylidyne, and metal-alkylidene precursors have proved useful. In addition, approaches involving pentadienyl reagents as the source of ring carbons have yielded notable successes. Vinylcyclopropene reagents have recently led to isolation of the first example of a metallabenzene valence isomer--a metallabenzvalene--and its subsequent conversion to a planar metallabenzene. Finally, interligand attacks of butadienyls on carbonyls have produced a variety of transient oxy- or alkoxy-substituted metallabenzene species. The development of new synthetic approaches, particularly systematic approaches that can be used with a variety of transition metals, is the key issue facing metallabenzene chemists. One hundred and thirty-five years after Kekulé's celebrated dream, aromatic chemistry continues to be a fascinating and provocative research topic. Metallabenzenes represent one of the "new frontiers" that promise to keep aromatic chemistry vibrant well into the 21st century.