Ring opening of a strained chelate complex of osmium has been investigated as a means to transient coordinative unsaturation under mild conditions in normally kinetically inert, third-row, platinum-group metals. cis-L4Os(H)(NC5H4X) (L = PMe3; X = O (1), NH (2), S (3)) were prepared by treatment of fac-L3Os(H)(η2-CH2PMe2) with 2-(HX)C5H4N. An X-ray structure determination establishes 1 to be the κN tautomer with the keto group pointed away from the hydride and toward the PMe3 ligand trans to hydride. NMR spectra support the κN structure in solution. Which nitrogen is coordinated in 2 is not established, but NMR supports the κS tautomer of 3 in solution. At 80 °C in C6D6, 1 incorporates L‘ (P(CD3)3) in a first-order reaction (t1/2 ≈ 14 min) stereospecifically at site b (trans to hydride) with ΔH⧧ = 26.9 ± 1.4 kcal/mol and ΔS⧧ = 3.8 ± 4.4 eu for the substitution. Complex 2 at 80 °C also exchanges with L‘ only in site b, but more slowly than 1 (t1/2 ≈ 3.5 h). Sublimation of 1 at 55 °C under vacuum yields mer-L3Os(H)(NC5H4O-κ2N,O) (mer-4), believed to have mer-4-(HON) geometry (H trans to nitrogen). In toluene at reflux 1 loses L, forming fac-4. Heating mer-4 in benzene 3 h at 110 °C forms fac-4. At 22 °C, L‘ adds to fac-4 with t1/2 ≈ 3 h, generating pure (La‘)(La)(Lb)(Lc)Os(H)(NC5H4O-κN), which then undergoes the much slower exchange with L‘ typical of 1 specifically forming (La‘)(La)(Lb‘)(Lc)Os(H)(NC5H4O-κN). Reaction of mer-4 with excess L‘ in ca. 2 h generates trans-L3L‘Os(H)(NC5H4O) then trans-L‘4Os(H)(NC5H4O) over 10 h, and finally cis-L‘4Os(H)(NC5H4O-κN) over 10 days. A scheme is presented that correlates all of the above transformations. Several reactions of fac-4 were examined. fac-4 and CO2 (10 atm, benzene-d6, 80 °C, 1 h) afforded fac-L3Os(κ1-O2CH)(κ2-NC5H4O), 5. Heating pure 5 in C6D6 (110 °C, 2 h) regenerated 4. Reaction of fac-4 with D2 gas (0.84 atm, C6D6, 80 °C) caused formation of Os-D with no other changes. fac-4 in neat 1-hexene (80 °C, 17 h) gave fac-L3Os(n-hexyl)(κ2-NC5H4O) (6) quantitatively, while fac-4 under 1 atm of ethylene (C6D6, 80 °C, 36 h) gave fac-L3Os(ethyl)(κ2-NC5H4O) (7) quantitatively. Heating 6 in C6D6 at 80 °C for 48 h gave fac-4 back again with a mixture of hexene isomers with a 2:1 terminal:internal alkene ratio. Heating 7 (C6D6, 80 °C) also regenerated 4 and ethylene. Heating at 80 °C for 7 days of a sample containing 1-hexene in C6D6 solvent and 5 mol % of fac-4 under 1 atm of hydrogen gas quantitatively gave hexane, while 4 remained unchanged. This is a substantial increase in reactivity over L4Os(H)2 through the use of the strained chelate.