The investigation on the synthesis and reactivity of metal vinylidene complexes has always been an active area, especially given their applications as well-known catalysts for metathesis and carbon–carbon and carbon–heteroatom coupling reactions. 2] Compared with other reactions of metal vinylidenes, the cyclization reactions are underdeveloped. Previous studies showed that the vinylidene M=Ca bond participated in cycloadditions (see a) as key steps in carbon– carbon coupling reactions, and some cycloaddition products have also been isolated. Interestingly, similar reactions of the metal vinylidene Ca=Cb bond (see b) have been realized as well. [5] However, cyclizations of the skeleton of metal vinylidene (M=Ca=Cb) as a unit (see c) remain scarce. In fact, the reported cyclization is a formal [3+2] cycloaddition reaction. To the best of our knowledge, the [3+3] cyclization reactions of metal vinylidene to construct six-membered rings have not been reported. Moreover, since the first isolation of metallabenzene was achieved by Roper and co-workers, considerable research interest has been attracted in this field. Nevertheless, the metallabenzene analogue isometallabenzene, which can be seen as a metal-containing 1,2,4-cyclohexatriene, has not been thoroughly explored. Until now, only the 16e isometallabenzene [Os{=C=C(Ph) CH(Ph) CH=C(CH2Ph)}Cl(PiPr3)2], prepared by double coupling reactions, was reported by Esteruelas and co-workers in 2004. Herein, we report an unprecedented formal [3+3] cycloaddition reaction between the hydride vinylidene complex [OsHCl2{=C=CH(PPh3)}(PPh3)2] (2) and alkynols under mild conditions to give stable 18e iso-osmabenzenes. The origin of the unexpected stability of these iso-osmabenzene complexes and their isomerization into h-cyclopentadienyl complexes through metalated cyclopentadiene intermediates is also described. Stirring of the osmium hydride alkenylcarbyne complex [OsHCl2( C C(PPh3)=CHPh)(PPh3)2]BF4 (1) in a mixture of H2O/CH3OH (3:2 v/v) at reflux produced the osmium hydride vinylidene complex 2, which was isolated as a lightyellow solid in 60% yield (Scheme 1). The other product, PhCHO, was detected by H NMR (d= 9.3 ppm; PhCHO) and GC analysis of the reaction mixture (see Figure S1 in the Supporting Information). Complex 2 was characterized by NMR spectroscopy and elemental analysis, and the structure was additionally confirmed by single-crystal X-ray diffraction (Figure S2).