Metallabenzenes [1] are species which have attracted ample interest from the academic community because their structures, [2] electronic properties, [3] and reactivity. [4] Despite recent significant advances in this field, which have been reviewed lately, [1b,c] and the development of different synthetic methods, a parent metallabenzene, that is, a complex in which apart from the metal center, only CH units are in the aromatic six-membered ring, remains an unknown and appealing target. This situation contrasts with the related phosphabenzene and arsabenzene, for which corresponding parent structures were reported several decades ago. [5] We have already described [6] a new synthetic route to iridabenzene units that relied on the reaction of fully CO 2 Me substituted Tp Me2 -iridacyclopentadienes (Tp Me2 = hydrotris(3,5-dimethylpyrazolyl)borate) with a terminal olefin, such as propene (Scheme 1 a). We explained iridabenzene formation on the basis of a rare olefin-to-alkylidene isomerization, [7] followed by migratory insertion and a-H elimination processes. Interestingly, C 2 H 4 did not afford a related methyl-substituted iridabenzene but inserted instead into an Ir À C bond giving a H 2 O-stabilized iridacycloheptadiene. [8] In parallel studies, [9] we also demonstrated that an Ir III complex of a doubly metallated Tp Mes ligand (Tp Mes = hydrotris(3-mesitylpyrazolyl)borate) cleaved dichloromethane cleanly to produce a monochloro carbene ligand, :C(H)Cl, with the remaining Cl and H atoms adding to the metal and to one of the Ir À CH 2 bonds of the starting compound, respectively (Scheme 1 b).Encompassing these two reactivity modes, we now report the formation of the first parent metallabenzene (complex 2 in Scheme 1 c) in the reaction of the compound [TpIr(C 2 H 4 ) 2 ] (1) [10] (Tp = hydrotris(pyrazolyl)borate) with acetylene in CH 2 Cl 2 .A solution of complex 1 (50 mg, 0.1 mmol) in CH 2 Cl 2 (20 mL) saturated with acetylene [11] (glass ampoule, À60 8C, 1 atm) was allowed to reacted from À60 8C to room temper-ature to yield quantitatively (by NMR spectroscopy) an approximately 1:1 mixture of the iridabenzene 2 and the bchlorinated vinyl hydride complex 3 (Scheme 2). Under the above conditions, the reaction outcome is highly reproducible but the yield of the iridabenzene is highly sensitive to deviations from them, for example, if C 2 H 2 saturation is carried out at À20 8C the ratio of complexes 2 and 3 becomes ca. 1:2.Complex 2 was purified by chromatography on silica gel and characterized by the usual spectroscopic and analytical methods, including single-crystal X-ray studies (Figure 1). The five carbon atoms of the metallacycle are almost coplanar, but the Ir atom is displaced from this plane by 0.29 . This deviation is a typical characteristic of metallabenzenes, [2] and has been documented for other iridaben-Scheme 1. Previous reactivity studies on Tp'Ir III systems (a), [6] and (b) [9] relevant to the iridabenzene synthesis reported herein (c). exc. = saturated.Scheme 2. Reaction of complex [TpIr(C...