The geometry and bonding nature of Cp(CO)(2)W(C=CH)(SiH2) (1) and the reaction leading to the formation of 1 from Cp(CO)(2)W(Si(H)(2)C CH) (9) were theoretically investigated with DFT, MP2 to MP4(SDTQ), and CCSD(T) methods, where 9 and 1 were adopted as models of the interesting new complexes reported recently, Cp*(CO)(2)W(Si(Ph)(2)C (CBu)-Bu-t) and Cp*(CO)(2)W(C (CBu)-Bu-t)(SiPh2), respectively. Our computational results clearly indicate that 1 involves neither a pure silacyclopropenyl group nor pure silylene and acetylide groups and that the silylene group strongly interacts with both the W center and the acetylide group. Frontier orbitals of 1 resemble those observed in the formation of silacyclopropene from silylene and acetylene. The frontier orbitals, as well as the geometry, indicate that the (CCH)(SiH2) moiety of 1 can be understood in terms of an interesting intermediate species trapped by the W center in that formation reaction. Complex 1 is easily formed from 9 through Si-C sigma-bond activation with moderate activation barriers of 15.3, 18.8, and 15.8 kcal/mol, which are the DFT-, MP4(SDTQ)-, and CCSD(T)-calculated values, respectively. This reaction takes place without a change of the oxidation state of the W center. Intermediate 9 is easily formed from Cp(CO)(2)W(Me)(H3SiC CH) via Si-H oxidative addition, followed by C-H reductive elimination. The bonding nature of 9 is also very interesting; the nonbonding pi-orbital of the H2SiCCH moiety is essentially the same as that of the propargyl group, but the pi-conjugation between Si and C atoms is very weak in the pi-orbital, unlike that in the propargyl group.