Addition of 4-substituted pyridine derivatives (4-R-C5H4N; R = Me, i-Pr, NMe2) to the tetratolylporphyrinato (TTP) osmium carbene complex (TTP)Os═CH(CO2Et) affords stable osmium ylide complexes, trans-(4-R-C5H4N)(TTP)Os-CH(CO2Et)(4-R-C5H4N). Dynamic NMR studies of the 4-picoline and 4-(dimethylamino)pyridine adducts show that the free energies for the barrier to rotation around the Cα-N axis are respectively 14 ± 1 kcal/mol at 298 ± 5 K and 13.0 ± 0.5 kcal/mol at 268 ± 2 K. The barrier to rotation around the C- NMe2 axis of the (dimethylamino)pyridine ylide fragment is 13.0 ± 0.5 kcal/mol at 240 ± 2 K. Addition of 4-picoline (4-pic) to (TTP)Os═CH(SiMe3) leads to the equilibrium formation of trans-(4-pic)(TTP)Os═CH(SiMe3) and the osmium ylide complex trans-(4-pic)-(TTP)Os═CH(SiMe3)(4-pic). 1H NMR spin saturation transfer experiments confirm the existence of this equilibrium. Disubstituted carbene complexes (TTP)Os═CR2 do not yield osmium ylide complexes upon treatment with 4-picoline. Prolonged treatment of (TTP) Os═C(CO2Et)2 with 4-picoline affords mainly (TTP)Os(4-pic)2. The bis(picoline) complex (TTP)Os(4-pic)2 crystallizes in the monoclinic space group P21/n with 1.74 molecules of benzene (C60H50N6Os·1.7C6H6, a = 10.800(2) Å,b = 21.332(2) Å,c = 12.868(1) Å, β= 108.89(1)°, V = 2805 Å3, Z = 2, R = 3.08%, Rw = 3.80%). Key metrical parameters include an Os-Npic distance of 2.078(3) Å and a torsional angle between the plane of the axial ligand and the closest Os-Npyrrole bond of 40.1(3)°. Treatment of the osmium ylide complexes with styrene yields cyclopropane derivatives with high stereoselectivities and conversions ranging from 65 to 95%. The trans/cis ratios for these reactions range from 23 to 28.