Activation of H−H, Si−H, and C−H Bonds by<i>n</i>s<i>n</i>p Excited States of Metal Atoms

Abstract

We present a comprehensive overview of our current knowledge of the interactions of valence M(nsnp 3P) and M(nsnp 1P1) 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(nsnp 3P1) 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(npπ)−XH(σ*) donor−acceptor molecular orbital interactions, since the Si−H bond length in SiH4 is ∼1.5 Å compared to C−H bond lengths of ∼1.1 Å. It is also concluded that Mg(3s3p 1P1) and Zn(4s4p 1P1) 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(npπ)−XH(σ*) energy matches and overlap, which overcomes M(ns)−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(1P1) versus Zn(1P1) 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(1P1) state (but not the Mg(1P1) 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(nsnp 1P1) states [as well as O(1D2)] of H−H, Si−H, and C−H bonds probably does not occur.