DFT calculations clearly show that a zirconocene-ethylene complex, CP2Zr(C2H4), easily reacts with hydrosilane, SiH4, to afford either Cp2Zr(H)(C2H4SiH3) or CP2Zr(SiH3)(C2H5). In the transition state, the Si-H anti-bonding sigma*-orbital of SiH4 overlaps well with the it-back donating molecular orbital between Zr d(pi) and C2H4 pi* orbitals.