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-L 4 Os(H)(NC 5 H 4 X) (L ) PMe 3 ; X ) O (1), NH (2), S (3)) were prepared by treatment of fac-L 3 Os(H)(η 2 -CH 2 PMe 2 ) with 2-(HX)C 5 H 4 N. An X-ray structure determination establishes 1 to be the κN tautomer with the keto group pointed away from the hydride and toward the PMe 3 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 C 6 D 6 , 1 incorporates L′ (P(CD 3 ) 3 ) in a first-order reaction (t 1/2 ≈ 14 min) stereospecifically at site b (trans to hydride) with ∆H q ) 26.9 ( 1.4 kcal/mol and ∆S q ) 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 (t 1/2 ≈ 3.5 h). Sublimation of 1 at 55 °C under vacuum yields mer-L 3 Os(H)(NC 5 H 4 O-κ 2 N,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 withReaction of mer-4 with excess L′ in ca. 2 h generates trans-L 3 L′Os(H)(NC 5 H 4 O) then trans-L′ 4 Os(H)(NC 5 H 4 O) over 10 h, and finally cis-L′ 4 Os(H)(NC 5 H 4 O-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 CO 2 (10 atm, benzene-d 6 , 80 °C, 1 h) afforded fac-L 3 Os(κ 1 -O 2 CH)-(κ 2 -NC 5 H 4 O), 5. Heating pure 5 in C 6 D 6 (110 °C, 2 h) regenerated 4. Reaction of fac-4 with D 2 gas (0.84 atm, C 6 D 6 , 80 °C) caused formation of Os-D with no other changes. fac-4 in neat 1-hexene (80 °C, 17 h) gave fac-L 3 Os(n-hexyl)(κ 2 -NC 5 H 4 O) (6) quantitatively, while fac-4 under 1 atm of ethylene (C 6 D 6 , 80 °C, 36 h) gave fac-L 3 Os(ethyl)(κ 2 -NC 5 H 4 O) (7) quantitatively. Heating 6 in C 6 D 6 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 (C 6 D 6 , 80 °C) also regenerated 4 and ethylene. Heating at 80 °C for 7 days of a sample containing 1-hexene in C 6 D 6 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 L 4 Os(H) 2 through the use of the strained chelate.