Activation of H−H, Si−H, and C−H Bonds bynsnp Excited States of Metal Atoms

We present a comprehensive overview of our current knowledge of the interactions of valence M( n s n p 3 P) and M( n s n p 1 P 1 ) excited states with H−H, Si−H, and C−H bonds, where M = Mg, Zn, Cd, and Hg. It is proposed that the high reactivity of M( n s n p 3 P 1 ) states with H−H and Si−H bonds compared to C−H bonds is due to the lack of steric hindrance in the localized, side-on, M( n pπ)−XH(σ*) donor−acceptor molecular orbital interactions, since the Si−H bond length in SiH 4 is ∼1.5 Å compared to C−H bond lengths of ∼1.1 Å. It is also concluded that Mg(3s3p 1 P 1 ) and Zn(4s4p 1 P 1 ) efficiently activate C−H bonds as well as H−H and Si−H bonds not just because of their higher energy but because of better M( n pπ)−XH(σ*) energy matches and overlap, which overcomes M( n s)−XH(σ) repulsion and the steric hindrance. It is further proposed that the striking differences in the microscopic mechanisms of attack of C−H bonds by Mg( 1 P 1 ) versus Zn( 1 P 1 ) may be due to the fact that the Zn(4s) “core” is substantially (∼0.2 Å) smaller than the Mg(3s) “core”, allowing true insertion of the Zn( 1 P 1 ) state (but not the Mg( 1 P 1 ) state) into C−H bonds to form (by surface hopping) long-lived ground-state zinc alkyl hydrides which decompose in a non-RRKM fashion to yield the observed ZnH product. Finally, the experimental results to date (as well as ab initio calculations) indicate that direct, end-on “abstractive” attack of M( n s n p 1 P 1 ) states [as well as O( 1 D 2 )] of H−H, Si−H, and C−H bonds probably does not occur.