Theoretical Study of Tungsten eta~3-Silaallyl/eta~3-Vinylsilyl and Vinyl Silylene Complexes: Interesting Bonding Nature and Relative Stability

摘要

The geometries and bonding nature of interesting new tungsten eta~3-silaallyl/eta~3-vinylsilyl complex Cp-(CO)_2W(eta~3-H_2SiCHCH_2) (1) and tungsten vinyl silylene complex Cp(CO)_2W(CHCH_2)(SiH_2) (2) and the conversion reaction of 1 to 2 were theoretically investigated with the density functional theory (DFT) and CCSD(T) methods, where 1 was adopted as a model of Cp*(CO)_2W(eta~3-Me_2SiCHCMe_2). The nonbonding pi orbital (phi_(n pi)) of the eta~3-H_2SiCHCH_2 group is- similar to that of the eta~3-allyl group except that the Si p orbital more contributes to phi_(n pi) than the C p orbital. On the other hand, the pi orbital (phi_(pi)) of the eta~3-H_2SiCHCH_2 group is considerably different from that of the eta~3-ailyl group; the pi-conjugation between the Si and C atoms is very weak, unlike that of the eta~3-allyl group in which pi-conjugation is considerably strong. Thus, 1 can be understood to be a species between tungsten eta~3-vinylsilyl and tungsten eta~3-silaallyl complexes. From the geometry and frontier orbitals, 2 can be understood to be a tungsten vinyl silylene complex in which charge transfer interaction between the silylene and vinyl groups is very weak. Complex 1 is much more stable than 2 by 21.0 (20.9) kcal/mol, but Cp(CO)_2W(eta~3-H_2SiCCH) (3) is less stable than Cp(GO)_2W(CCH)(SiH_2) (4) by 0.7 (4.9) kcal/mol, where the CCSD(T)- and DFT-calculated values are given without and in parentheses, respectively. This means that the tungsten eta~3-silaallyl/eta~3-vinylsilyl complex can be isolated but the tungsten vinyl silylene complex cannot, unlike the tungsten acetylide silylene complex Cp*(CO)_2W(CC~tBu)(SiPh_2) which was isolated recently. Complex 1 converts to 2 with a large activation barrier of 34.2 (33.2) kcal/mol, while 3 easily converts to 4 with a moderate activation barrier of 15.8 (15.3) kcal/mol. These differences between 1 and 3 can be interpreted as follows: Though the Si-C bond is weak in 1, the W-(eta~3-H_2SiCHCH_2) interaction is considerably strong. Moreover, the W-vinyl and silylene-vinyl interactions are very weak in 2. On the other hand, the Si-C bond is strong but the W-(eta~3-H_2SiCCH) interaction is weak in 3. Moreover, the W-acetylide and silylene-acetylide interactions are very strong in 4. The reasons are discussed in detail.