The reactions of Cd(S-1:5s(2), P-3,P-1:5s(1)5p(1)) and Hg(S-1:6s(2), P-3,P-1:6s(1)6p(1)) with SiH4 have been studied through multiconfiguration self-consistent-field (MCSCF) (with relativistic effective core potentials) followed by extensive variational and perturbational second-order multireference Moller-Plesset configuration interaction by perturbation selected by iterative process (CIPSI) calculations using extended Gaussian basis sets. It was found that both metal atoms in their P-3(ns(1)np(1)) state break the Si-H bond of silane spontaneously, leading directly to the MH + SiH3 final products, in agreement with the experimental results of this reaction for Cd. One important qualitative difference between the Cd and Hg(P-3) reactions is that for the former an unstable intermediate was found, whereas for the latter no intermediate exists at all. Again, for both atoms, the P-1(ns(1)np(1)) state is also inserted in the Si-H bond and the corresponding interaction surface shows an avoided crossing with the lowest-lying X (1)A' potential surface, adiabatically correlated with the M(S-1:ns(2)) + SiH4 reactants. This interaction leads eventually to the MH + SiH3 products. The structure of these HMSiH3 intermediates, diabatically correlated with the M(P-1:ns(1)np(1)) + SiH4 reactants, was carefully studied, as well as the dissociation channels leading to the MH + SiH3 and H + MSiH3 products. Accurate energy differences between all these species are also reported. The theoretical results obtained for the mercury reaction are discussed in light of the very recent experimental results of Legay-Sommaire and Legay [J. Phys. Chem. A 102, 8579 (1998)] for the insertion of Hg(P-3:4s(1)4p(1)) in SiH4 over N-2 and rare gas matrices. Our results confirm their conclusion that the photochemical insertion of Hg(P-3) into the Si-H bond of silane proceeds without any activation barrier.