Intermolecular agostic interactions

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

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. 

There is much evidence that low-coordinate lithium centers found within certain organolithiums (RLi) (e.g., two-coordinate centers in rings with n = 2 or 3) will interact with C—H bonds within the R groups. Such agostic interactions can be intra- or intermolecular. 

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). 

Compound 64 is a true nickel(O) alkyne complex with no agostic interactions between Ni and the methyl groups of the ligands. In the crystal the molecules are associated by four intermolecular hydrogen-bond interactions between neighboring butynediol ligands. This leads to the formation of chain polymers. 

Now, one must wonder if there is any limit to the ability of metals to bond in a stable way to other o bonds, including those in non-reactive molecules like alkanes. In fact, evidence for an intermediate methane complex has been found at low temperature in the reductive elimination of methane from a cationic rhenium methyl hydride [34]. The ab initio theoretical study of the intermolecular process of oxidative addition of a methane C - H bond has led to the location of transition states where the bond is partially broken [35]. The same results have been fond for intramolecular oxidative additions which are related to agostic interactions. In fact, agostic interaction itself is a kind of non-oxidative coordination [15,36]. For unsaturated substrates like ethylene, the activation of a C - H bond seems to follow an intermolecular path, without any previous coordination of the double bond. A feasible explanation consists here of the fact that metal orbitals suitable for ethylene coordination are the same as those which are responsible for oxidative addition, thus making the processes competitive [37]. 

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. 

Intermolecular hydrogen bonds with an electron-rich metal as acceptor, and NH as donor, have been recognized [76a]. The better-known agostic C-H - M interactions [76b-76d] are predominantly intramolecular. 

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