The investigation on the synthesis and reactivity of metal vinylidene complexes has always been an active area, [1] especially given their applications as well-known catalysts for metathesis and carbon-carbon and carbon-heteroatom coupling reactions. [1b,e, 2] Compared with other reactions of metal vinylidenes, the cyclization reactions are underdeveloped. Previous studies showed that the vinylidene M = C a bond participated in cycloadditions (see a) as key steps in carboncarbon coupling reactions, [3] and some cycloaddition products have also been isolated. [4] Interestingly, similar reactions of the metal vinylidene C a = C b bond (see b) have been realized as well. [5] However, cyclizations of the skeleton of metal vinylidene (M = C a = C b ) as a unit (see c) remain scarce. In fact, the reported cyclization is a formal [3+2] cycloaddition reaction. [5c] 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, [7a] considerable research interest has been attracted in this field. [6,7] 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 À isometallabenzene[Os{ = C = C(Ph) À CH(Ph) À CH = C(CH 2 Ph)}Cl-(PiPr 3 ) 2 ], prepared by double coupling reactions, was reported by Esteruelas and co-workers in 2004. [8] Herein, we report an unprecedented formal [3+3] cycloaddition reaction between the hydride vinylidene complex [OsHCl 2 -{=C=CH(PPh 3 )}(PPh 3 ) 2 ] (2) and alkynols under mild conditions to give stable 18e À iso-osmabenzenes. The origin of the unexpected stability of these iso-osmabenzene complexes and their isomerization into h 5 -cyclopentadienyl complexes through metalated cyclopentadiene intermediates is also described.Stirring of the osmium hydride alkenylcarbyne complex [OsHCl 2 ( C À C(PPh 3 ) = CHPh)(PPh 3 ) 2 ]BF 4 (1) [9] in a mixture of H 2 O/CH 3 OH (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 1 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).Complex 2 is thermally stable in the solid state but highly reactive in solution. Treatment of 2 with HC CCH(OH)Ph in dichloromethane or chloroform at room temperature led to a fast color change from light yellow to brown red. The unexpected iso-osmabenzene 3 was generated from a formal [3+3] cycloaddition and isolated in high yield (93 %). Similarly, when 2 reacted with HC CCH(OH)CH = CH 2 or HC CCH(OH)CH 2 CH 3 under the same reaction conditions, the other two iso-osmabenzenes, 4 (88 % yield) and...