Agostic Si-C bonds

Hence also the interest in finding agostic bonds without H atom such as C-C and Si-C bonds. Very few examples of agostic C-C bonds are known [24-26] but many examples of agostic Si-C bond have been established especially with the early transition metals [27-31]. 

Also the salt-free and solvent-free mono Cp lanthanide alkyl complex Cp La[CH-(SiMe3)2]2 has been synthesized and the X-ray structure together with that of its THF precursor show the stabilization of the unsaturated lanthanum center by unusual agostic Si-C bonds. The removal of THF produces a different conformation of the disylil ligands and pyramidalization of the Cp La [CH(SiMe3)2]2 moiety. 

In hypersilyl sodium and potassium both, infra- and intermolecular M "H-C agostic interactions occur, leading to short M -C distances. Interactions of this type were found in many other alkali metal compounds - a survey is given in reference [5]. In the hypersilyl lithiiun dimers only short znframolecular M--H-C contacts are observed (M "C 239, 245 pm), combined with a significant lengthening of the involved Si-C bonds (191 pm) and a pronounced tilting of the hypersilyl ligands (Fig. 3). 

Several reviews on silane organometallic chemistry have been written, both on experimental [65-67] and theoretical aspects [68]. The agostic Si-H bond has been shown in early [69-88] and late [89-96] transition metal complexes in a wide variety of situations. The Si-H bond is a better candidate for an agostic interaction than C-H because it is more polarisable and the H is more hydridic. Computational studies confirm the presence of an M H-Si interaction in various systems. A recent review summarizes the computational studies up to 2002 [68]. It is interesting to include the o complexes. For instance the two hexacoordinated d complexes, 16 (characterized by X-ray [97]) and 17 (characterized by NMR [98]), probably have similar electronic M- Si-H interactions although no calculations have yet been carried out to test this. The difference between the two complexes could thus be mostly due to the fact that the Si-H bond is forced to remain in close vicinity to the metal in 16 because the alkene does not dissociate easily. This factor (essentially entropic) allows the experimental observation of weaker interactions. 

Alkynylsilanes C=C—SiH Acetylene exchange in complexes 1 and 3 by the alkynylsilanes RC=CSiMe2H (R = 7Bu, Ph, SiMe3, SiMe2H) yields complexes in which an agostic interaction between the Si—H bond and the metal center is indicated for complex 100, but not for 101 [53],... 

The silanol complex 57 exhibits a Si H M agostic interaction characterized by a /(Si-H) of 41 Hz and a Si-H distance of 1.70(7) It would be incautious to interpret such a low value of the Si-H coupling in terms of a significant Si-H bond activation, because the Si-H bond forms rather acute angles with the Si-C and Si-Si bonds (about 82 and 101°, respectively) and thus must have a considerable p character on silicon, which should contribute to the decrease of /(Si-H). The silanol ligand is -coordinate to ruthenium and the Ru-Si bond of 2.441(3) A is not exceptional, but the Si(SiMe3)3 deviates from the silanol plane by 19.0°, probably as a result of the Si-H interaction. Deprotonation of 57 by strong bases affords a neutral ruthenocene-like product. 

Two molecules can be combined to form an ion-pair through a a coordination bond, in which one molecule provides its X-H (X = B, C, N, O, Si) a bonding electrons to a transition metal atom (such as Zr) of another molecule. A good example is [(C5Me5)2Zr+Me][B Me(C6F5)3], whose structure is shown in Fig. 11.5.5. This bonding type is called an intermolecular pseudo-agostic (IPA) interaction. 

---