AB-INITIO MOLECULAR-ORBITAL STUDY OF ELECTRONIC AND GEOMETRICAL STRUCTURES OF MCH(2)(-2, WHERE M=CO, RH, AND IR() COMPLEX AND ITS REACTIVITY WITH H)

By using CASSCF (for optimization of geometries) and MR-SDCI-CASSCF (f or energies) methods we have studied and compared the mechanism of rea ction MCH(2)(+) + H-2, as well as the electronic and geometrical struc ture of the MCH(2)(+) complex, where M = Co, Rh, and Ir. It has been f ound that the mechanisms of reaction MCH(2)(+) + H-2 --> M(+) + CH4 (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(+) + C H4. The key step is activation of the H-H bond, which has a barrier ab out twice as high for M = Co as for M = Rh; reaction (1) occurs more e asily for M = Rh than M = Co. M = Ir completely changes the mechanism of reaction (1), which now follows the path: IrCH2+((3)A(2)) + H-2 --> (H-2)IrCH2+((3)A(2)) --> [TS1, H-2-activation] --> (H)(2)IrCH2+((1)A' ) --> [TS2, H-migration] --> HIrCH3+((3)A) --> [TS3, CH4-elimination] --> IrCH4+((3)A(2)) --> Ir+(F-5, s(1)d(7)) + CH4. The reaction (1) is exothermic for M = Co and Rh, but endothermic for M = Ir. For M = Co a nd Rh, the reverse reaction M(+) + CH4 can give only one product MCH(4 )(+) and does not proceed further easily; for M = Co, at elevated temp erature CoCH4+ may give CoH+ and CoCH3+. However, for M = Ir the rever se reaction can proceed further to give hydridomethyl HIrCH3+ and bish ydrido (H)(2)IrCH2+ complexes, as well as IrCH4+.