SCF and electron correlation calculations are reported for the lowest singlet and triplet state of the title compounds. Geometry optimization and the determination of the complete harmonic force field has been performed within the Hartree-Fock method using flexible basis sets including d functions on the heavy atoms. Electron correlation energies were calculated by the CEPA-2 approach. The effect of electron correlation on CC, CSi, and SiSi bond distances and on the pertaining force constants is discussed. For fixed geometries large-scale CEPA calculations with 2d sets on the heavy atoms and one p set on the hydrogens were performed. Single-triplet splittings and relative stabilities of the various isomers were computed.