Square-planar allcylidyne and five-coordinate alkylidene mixed iPr(3)P-Os-IPr (IPr = 1,3-bis(diisopropylphenyl)imidazolylidene) complexes have been diScoverect and characterized, and their formation has been rationalized. The cationic five-coordinate hydride-allcylidyne compounds [OsHX( CPh)(IPr)((PPr3)-Pr-i)]OTf (X = Cl (1), F (4); OTf = CF3SO3) undergo deprotonation with (KOBu)-Bu-i to afford the transhalide-allcylidyne square-planar derivatives OsX( CPh)(IPr)(PiPr3) (X = Cl (2), F (5)). Oxidative addition of the C(sp)-H bond of phenylacetylene and methyl propiolate along the CI-Os-CPh axis of 2 with the hydrogen atom directed to the alkylidyne leads to allcynyl-cis-hydride-alkylidyne intermediates, which rapidly evolve into the five-coordinate alicylidene complexes Os(C CR)C1( CHPh)(IPr)(PiPr3) (R = Ph (6), CO2Me (7)) as a consequence of the migration of the hydride from the metal center to the C-alpha atom of the allcylidyne. Oxidative addition of the C(sp)-H-bond of methyl propiolate along the X-Os-CPh axis of 2 and 5 with the hydrogen atom directed to the halide gives the alltynyl-trans-hydride-alkylidyne derivatives OsH(C CCO2Me)X( CPh)(IPr)(P(i)Pr3) (X = Cl (8), F (9)). Complex 8 evolves into 7. However, complex 9 containing the stronger x-donor fluoride is stable. The oxidative addition of HC1 to 2 selectively yields the cis-hydride-allcylidyne compound OsHCl2( CPh)(1Pr)(PPr3) (10), which is also stable.