The first nsnp P-3(1) excited states of Cd and Zn atoms are shown to readily activate Si-H bonds in SiH4. The nascent quantum state distributions of the CdH(v;N) and ZnH(v;N) products in the reactions of Cd(P-3(1)) and Zn(P-3(1)) with SiH4 have been determined using the laser pump-probe technique. The results are discussed within the context of our current knowledge about the chemical interactions of valence M(nsnp P-3) excited states with Si-H, H-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 simply 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.