Ab Initio Molecular Orbital Study of Electronic and Geometrical Structures of MCH 2 + Complex and its Reactivity with H 2 , Where M = Co, Rh, and Ir

By using CASSCF (for optimization of geometries) and MR‐SDGI‐CASSCF (for energies) methods we have studied and compared the mechanism of reaction MCH 2 + + H 2 , as well as the electronic and geometrical structure of the MCH 2 + complex, where M = Co, Rh, and Ir. It has been found that the mechanisms of reaction MCH 2 + + H 2 → M + + CH 4 (1) for M = Co and Rh are similar and follow the path: MCH 2 + +H 2 → (H 2 )MCH 2 + → [TS1, H 2 ‐activation] → MCH 4 + → M + + CH 4 . The key step is activation of the H‐H bond, which has a barrier about twice as high for M = Co as for M = Rh; reaction ( 1 ) occurs more easily for M = Rh than M = Co. M = Ir completely changes the mechanism of reaction ( 1 ), which now follows the path: IrCH 2 + ( 3 A 2 ) + H 2 → (H 2 ) IrCH 2 + ( 3 A 2 ) → [TS1, H 2 ‐activation] → (H) 2 IrCH 2 + ( 1 A') → [TS2, H‐migration] → HIrCH 3 + ( 3 A) → [TS3, CH 4 ‐elimination] → IrCH 4 + ( 3 A 2 ) → Ir + ( 5 F, s 1 d 7 ) + CH 4 . The reaction ( 1 ) is exothermic for M = Co and Rh, but endothermic for M = Ir. For M = Co and Rh, the reverse reaction M + + CH 4 can give only one product MCH 4 + and does not proceed further easily; for M = Co, at elevated temperature CoCH 4 + may give CoH + and CoCH 3 + . However, for M = Ir the reverse reaction can proceed further to give hydridomethyl HIrCH 3 + and bishydrido (H) 2 CH 2 + complexes, as well as IrCH 4 + .