The potential energy surfaces for the unimolecular rearrangement XSidropP --> TS --> SidropPX were investigated using B3LYP and CCSD(T) methods. To explore electronic and steric effects on the relative stability of XSidropP and Si=PX, the first-row substituents X = H, Li, BeH, BH2, CH3, NH2, OH, and F have been used for this study. Our theoretical findings suggest that highly electronegative substitution at silicon can greatly stabilize the triple-bonded XSidropP species relative to the double-bonded Si=PX isomer, from both kinetic and thermodynamic viewpoints. [GRAPHICS] ints. [GRAPHICS]