The reactions of Zn(S-1, P-3, and P-1) with SiH4 have been studied through multiconfigurational self-consistent field (with relativistic effective core potentials) followed by extensive variational and perturbational second-order multireference Moller-Plesset configuration interaction by perturbation selected by iterative process calculations using extended Gaussian basis sets. The Zn atom in the P-3(4s(1)4p(1)) state breaks the Si-H bond of silane spontaneously, leading directly to the ZnH+SiH3 final products, in agreement with experimental results. The P-1(4s(1)4p(1)) Zn atom 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 Zn(S-1:4s(2))+SiH4 reactants. This interaction leads also to the ZnH+SiH3 products. The structure of the HZnSiH3 intermediate product was carefully studied as well as the dissociation channels leading to the ZnH+SiH3 and H+ZnSiH3 products. Accurate energy differences between these species are also reported. The qualitative difference in the behavior of the 3P(4s(1)4p(1)) Zn reaction with methane and silane has been explained by analyzing the corresponding potential energy surfaces; the present results confirm the C-H bond steric hindrance hypothesis advanced by Wang et al. [J. Chem. Phys. 104, 9401 (1996)]. (C) 1997 American Institute of Physics. [S0021-9606(97)03441-7].