Abstract: The HF/6-31¹⁺G^*basis set was used to study the characteristics of molecular orbitals and bond dissociation energies of the complexes, MSiH₂, of the first-row transition metals with SiH₂. The atoms in MSiH₂ are all coplanar, and M—Si bonds of ground state ³TiSiH₂ and ⁴CoSiH₂ show obviously the double bond characteristic. In ⁴VSiH₂ and ⁵CrSiH₂ there is the week single occupiedπ bond. The ⁸MnSiH₂ with a high-spin eightet ground state has both of single occupiedσ andπ bonds, and chief component of itsσ andπ orbital are the Mn₄s and the Si 3p_x orbitals, respectively. Other ground state MSiH₂ have only a double occupiedσ single bond. The most of M—Si bonds are quite covalent. The formation of M—Siσ bonds can be considered as the interaction between one electron in₄s or 4p_z, or 3d orbital of Mand other electron in Sisp hybrid orbital. The double occupiedπ bonds are also considered as the pairing of oneπ electron of SiH₂(³B₁) with one 3d electron of M. All of the HSiHbond angles in MSiH₂ are closed to that in SiH₂(³B₁). It means that the Si in MSiH₂ still keeps asp hybrid orbital similar to Si in SiH₂(³B₁ states). The Sc—Si, V—Si and Ni—Si bonds are longer because their formation involves mainly the 4s orbitals of M. On the contrary, the Cu—Si bond is shorter since itsσ bond would involve primarily the M 3d_Z² obitals. The ³TiSiH₂ and ⁴CoSiH₂ have much shorter bond lengths and larger bond dissociation energies because of their double bond character. From Sc to Cu, the bond dissociation energies of M—Si change in periodic trend. There is an approximate linear relationship between the bond dissociation energies and the metal ion promotion energy.

Key words: Transition-metal complexes, Bond dissociation energy, Silylene, Ab initio