Agostic C-H-M bond

Arene)(e/ido-dicyclopentadiene)M(0) complexes [M = Ru (211), Os (212)] are prepared by reduction of lc or Id with 2-propanol and Na2C03 in the presence of dicyclopentadiene (Scheme 15). Protonation of derivatives 211 and 212 with HPF6 leads to cations 213 and 214 which are stabilized by an agostic C—H—M bond, as shown by the X-ray structure of the osmium species (214). The cations 213 and 214 react with CN(t-Bu) to give complexes of type 215 in which the agostic C—H—M is no longer observed (102). 

Scheme VI. 1. Examples of complexes containing agostic C-H — M bonds.

Finally let us recall that the agostic C-H-M bond and the ligands H2 and H-SiRs also are weak ligands through their o bond. 

Reviews on transition-metal-alkyl bond dissociation energies and the formation of C-H bonds by reductive elimination are also relevant to the mechanisms of catalytic processes. It has been proposed that [Ti(Me2PCH2CH2PMe2)(Et)Cl3] contains a direct-bonding interaction between the titanium atom and the j8-C-H system the evidence for such agostic C-H-M bonds in other systems has been reviewed. Obviously, such bonds have important implications in alkane activation and it is particularly exciting that kinetic studies of the exchange reaction shown in equation (1) indicate that a bimolecular pathway predominates. ... 

Room-temperature spectra of species on Ni and Pd evaporated films that were also deposited at room temperature showed spectra from a mixture of ethyne-like species, probably 7r-bonded, and alkyl species with soft rCH modes indicating agostic C-H-M interactions. 

Although C—H bonds are relatively inactive, hydrogen atoms of coordinated alkyl species do interact with the metal center [66]. This three-way C—H—M bond is called an agostic interaction (from the Greek ayooro, to hold on to oneself) [67]. Agostic interactions are known for many complexes (Figure 3.24) [68,69], and feature in several important elementary steps, such as reductive elimination. They are more common with d° metal centers [70]. 

It is to be emphasized that the nature of an agostic C—H—M interaction is different from that of a hydrogen bond. In the agostic case the C—H bond supplies electrons to the metal atom, whereas in a hydrogen bond, X—H—Y, the X—H partner seeks electrons on the responding atom Y. Put differently, agostic interactions are 3c-2e bonds, whereas hydrogen bonds are 3c-4e bonds. 

In cases where a metal center is electron deficient, C-H bonds from a coordinated ligand can, in some cases, interact with the metal forming an agostic C-H... M interaction. Structural studies of agostic interactions contribute to our understanding of the role of a metal atom in oxidative addition or reductive elimination processes involving a C-H bond (Scheme 2), in an obvious analogy with our earlier discussion of metal insertion into an H-H bond (Scheme 1). 

In Section 1.2 we noted that the bonding in CH5+ could be described in terms of three 2c-2e C-H bonds and one 3c-2e C—H—H bond. In Section 1.3 we noted that 3c-2e C—H—M bonds could account for agostic interactions between coordinatively unsaturated metal atoms and substituent alkyl groups, and indeed for metal-alkane a complexes. Similarly, 3c-2e M—C—M bonds... 

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. 

Since the metal centers in the agostic C-H-M system are essentially electrophilic, the C-H bond becomes more acidic. A wide range of pA a values are observed and in some cases strong bases are necessary to deprotonate the C-H bonds. A unique Co(III)-alkyl complex 24 was prepared by facile deprotonation of an agostic bond by methoxide ion (Eq. 2.28) [76]. 

H-CR3 (intramolecular C-H bond belonging to another ligand of the same metal by another bond) the C-H-M bond is called agostic (see Chaps 6.3,6.4 and 8.2)... 

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