Evidence for Direct trans Insertion in a Hydrido‐Olefin Rhodium Complex—Free Nitrogen as a Trap in a Migratory Insertion Process

Reduction of the hydrido chloride complex [Rh(H)Cl{CH 3 C(CH 2 CH 2 ‐P( t Bu) 2 ) 2 }] ( 4 ) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N 2 )‐{CH 3 C(CH 2 P( t Bu) 2 ) 2 }] (2) and the unusual low‐valent hydrido‐olefin complex, [RhH{CH 2 C(CH 2 CH 2 P( t Bu) 2 ) 2 }] (3) . In the presence of N 2 , complexes 2 and 3 are in equilibrium in solution; 2 is about 2.9kcalmol −1 more stable than 3 + N 2 . Both complexes co‐crystallize in the solid state; they occupy the same crystallographic site in the crystal lattice ( P 2‐(1)/c; Z = 4; a = 12.173(2), b = 14.121 (3), c = 15.367 (3); α = 90, β = 106.50(3), γ = 90°). The mechanism of the reversible interconversion of 2 and 3 has been studied in detail. Complex 3 undergoes rapid olefin insertion/β‐hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized late transition state (δS≠≈ – 40 e.u.), which can be caused by a concerted “ trans migration”. This theoretically unfavorable process is assisted by a distortion from the ideal square‐planar configuration, including a decrease of the P‐Rh‐P angle and some bias of the double bond toward the hydride as indicated by the X‐ray crystal structure of 3 . Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex 2 . The dinitrogen dissociation from 2 was found to be the rate‐determining step for the overall interconversion of 2 and 3 (δ G ≠ 298 = 24.1 kcalmol −1 ).