Agostic interactions molecules

Figure 36 The structure of [(tBuN(H))4(tBuO)4Li4K4]-3(C6H6) 409. Benzene molecules and hydrogen atoms, except N-H have been omitted for clarity. Agostic interactions between potassium and feuO are not shown.

Recent experimental developments in coupling IR spectroscopy techniques with mass spectrometry, which allow the structural characterization of isolated and microsolvated protonated aromatic molecules in the gas phase have been summarized.64 Hydrogen-deuterium exchange has been observed at Ha in (59) and some closely related substrates when this ligand is complexed to Cu1 in [2Hg]acetone.65 The process is finely controlled by the precise coordination distance required to form agostic interaction between Cu(I) and the C-H bond and is believed to involve the enol form of [2H6]acetone and a reversible Cu(I) to Cu(III) interconversion. 

Agostic interactions involving phosphines are quite significant because they possess residual binding sites for other small molecules such as H2, and phosphine groups play crucial roles in many homogeneous catalysts. Figure 11.5.8(a)... 

It has also been suggested that the experimental barrier of activation for propagation in olefin polymerisation is due to rearrangement of y-agostic species into those with a-agostic interactions in order to free the coordination site for the insertion of the next monomer molecule, rather than the actual insertion (Figure 3.19) [358],... 

In the first step, a cis 1,2-insertion of cyclopentene takes place the intermediate derived from this insertion is sterically too demanding to permit insertion of another monomer molecule and is isomerised before another cyclopentene molecule is inserted. Since only /1-hydrogen atoms are available for the stabilisation of the active species (jS-agostic interaction), /1-hydride transfer, followed by monomer rotation or migration, and then reinsertion occur, leading to 1,3-linked polymer chains. The rotation around the Zr olefin n bond results in the formation of cis-1,3-poly(cyclopentylene) [scheme (3a)] [20] while the n face migration of the olefin across the nodal plane of the double bond results in the formation of trans-1,3-poly(cyclopentylene) [scheme (3b)] [19]. 

These adducts consist of cyclic dimers comprising folded M2Si2 backbones. The spectroscopic data reveal mainly ionic interactions between alkali metal cations and hypersilanide anions. Additionally, oligo-hapto coordination of the metal ions by the toluene molecules is observed. Intra- and intermolecular agostic interactions to C-H bonds finally govern the peculiarities of both, the molecular and the crystal structures of these compoimds. 

---