Molecular Orbital Study of the Reaction Mechanism of Sc+ with Methane. Comparison of the Reactivity of Early and Late First-Row Transition Metal Cations and Their Carbene Complexes

The reactions of Sc + + CH 4 and ScCH 2 + + H 2 have been studied with the CASSCF and MR-SDCI-CASSCF methods. The reaction of the ground state Sc + with CH 4 proceeds as Sc + ( 3 D, s 1 d 1 ) + CH 4 → ScCH 4 + ( 3 A‘‘) → TS2( 1 A‘) → HScCH 3 + ( 1 A‘) → TS1 → (H 2 )ScCH 2 + ( 1 A 1 ) → ScCH 2 + ( 1 A 1 ) + H 2 and is calculated to be endothermic by 24.8 kcal/mol. After formation of an ion−molecule complex ScCH 4 +, the reaction cannot proceed on the triplet surface because of a high barrier and has to cross over to singlet and reach the singlet transition state TS2 with a barrier of 28.6 kcal/mol. The insertion product HScCH 3 +, 4.9 kcal/mol more stable than the reactants Sc + + CH 4, produces most favorably an ion−molecule complex (H 2 )ScCH 2 + with a barrier of 28kcal/mol, which can dissociate to give H 2 and ScCH 2 + ( 1 A 1 ) with a small barrier of 3.2 kcal/mol. At higher temperatures, production of ScH + + CH 3 and ScCH 3 + + H from HScCH 3 + will dominate. The reverse reaction ScCH 2 + ( 1 A 1 ) + H 2 proceeds very easily and leads to HScCH 3 + ( 1 A‘), ScCH 4 + ( 3 A‘), and Sc + ( 1 D or 3 D) + CH 4 . Comparison of the results for Sc + (and ScCH 2 + ) with those for Fe + and Co + (and FeCH 2 + and CoCH 2 + ) shows the following. (a) Carbene complexes MCH 2 + of early transition metal cations, M = Sc, Ti, and V, should activate H−H/C−H bonds more easily than their late transition metal analogs and lead to hydridomethyl, HMCH 3 +, and ion−molecule M(CH 4 ) + complexes. For first-row late transition metals the hydridomethyl complex does not exist thermodynamically or kinetically. (b) The products of the reaction of early transition metal cations with methane at low temperatures should be MCH 2 + and H 2 as this channel (1) is less endothermic than MH + + CH 3 (2) and MCH 3 + + H (3) channels. For late transition metals, though the endothermicities are similar between three channels, channel 1 requires a large H−H bond activation barrier and does not take place, whereas channels 2 and 3 should be possible at high temperatures. All these similarities and differences in the reactivity of early and late first-row transition metal cations and their carbene complexes are explained by using a molecular orbital picture.