Activation of H-H, Si-H, and C-H bonds by nsnp excited states of metal atoms

We present a comprehensive overview of our current knowledge of the interactions of valence M(nsnp P-3) and M(nsnp P-1(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(nsnp P-3(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(np pi)-XH(sigma*) donor-acceptor molecular orbital interactions, since the Si-H bond length in SiH4 is similar to 1.5 Angstrom compared to C-H bond lengths of similar to 1.1 Angstrom. It is also concluded that Mg(3s3p P-1(1)) and Zn(4s4p P-1(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(np pi)-XH(sigma*) energy matches and overlap, which overcomes M(ns)-XH(sigma) 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(P-1(1)) versus Zn(P-1(1)) may be due to the fact that the Zn(4s) ''core'' is substantially (similar to 0.2 Angstrom) smaller than the Mg(3s) ''core'' allowing true insertion of the Zn(P-1(1)) state (but not the Mg(P-1(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 dale (as well as ab initio calculations) indicate that direct, end-on ''abstractive'' attack of M(nsnp P-1(1)) states [as well as O(D-1(2))] of H-H, SI-H, and C-H bonds probably does not occur.