Agostic interactions review

Numerous papers have appeared on further detailed studies of the mechanism, including chain transfer mechanisms, epimerization of the stereocenter shortly after its formation, the role of agostic interactions, etc. A special issue of Chemical Reviews gives a complete overview [29],... 

For a recent review on agostic interactions of early- to mid-transition metal methylidenes, see L. Andrews and H.-G. Cho, Organometallics, 2006, 25, 4040. 

These compounds are unique alkylating or arylating reagents and their reactivity has been reviewed [25,26], Some chemoselective reactions are presented. The addition reaction of aldehydes to 10 proceeds faster than that of ketones [23,25,27,28], Compound 8 reacts chemoselectively with an acetal, and not with an ester group [29], The structure of TiMeCl3(dmpe) (12) has been determined both by X-ray and neutron diffraction analysis. The latter confirms that there is a unique agostic interaction of the a-hydrogen atom with the titanium atom [30,31], This weak interaction stabi-... 

Abstract The agostic bond defines an intramolecular interaction where a a bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of a bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular, the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented. 

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. 

This article has reviewed the synthesis and reactivity towards small molecules of a range of U(lll) cyclooctatetraene and pentalene complexes. It is evident that in many cases the uranium centre is capable of tt back-bonding through the 5/ orbitals additionally, a C - C agostic interaction between a bound substrate and a U(IV) centre has been observed. Clearly uranium is capable of bonding with a degree of covalency , and this is perhaps why the reduction chemistry of U(III) is so rich and diverse, and not simply an iteration of low-valent lanthanide chemistry. 

Within this topic may be included agostic interactions and cyclometalation. A general review concerning the latter topic has appeared,as has a more mechanistically oriented review on the cyclometalation chemistry of aryl oxide ligands. ... 

These M-C-H interactions were named agostic interactions by Brookhart and Green [13] in 1983. They reported in detail on agostic interactions in their 1988 review entitled Carbon-Hydrogen-Transition Metal Bonds [14]. ... 

Brookhart and Green reported about 200 agostic interactions in their review. The ratios of the ligand groups in these 157 types of agostic interactions are shown in Table 6.3 ... 

Based on the data for about 200 agostic interactions in their review and the various types of agostic interactions shown in Fig. 6.3 [ 14], it was determined that these ligands initiate agostic interactions in metal atoms. In other words, it is considered that interactions of ligands with metal atoms activate the inert C-H bonds. 

In an earlier review the author published on the agostic interaction entitled Agostic Bonds in Cyclometalation in 2011 [23], cyclometalation reactions proceed extremely easily with a one-step reaction between metal compounds and substrates containing a heteroatom such as O, S, N, P, or As. Under mild reaction conditions, however, many agostic compounds, which are intermediates in these cyclometalation reactions with both transition metal and main group metal compounds, can be isolated. 

Recent developments in polymerisation by metallocene catalysts are found in general reviews w and more specialist monographs covering the role of agostic interactions, non-coordinating anions, applications to main-chain chiral polymers, and the analysis of polypropene by... 

Compounds which accommodate agostic interactions of Si-H moieties with transition metals may also be considered as a special type of higher-coordinated silicon compounds. A few reviews on this matter are also available [36]. These include a recent general overview [37] and further publications on selected sub-topics such as platinum compounds [38] or niobium and tantalum complexes [39]. Complexes of this type are also discussed in comparison to C-H and H-H interactions with lanthanides [40]. 

In these compounds the Si-H bcnid is likely to act as a o-donor to the transition metal (agostic interaction), or the Si-bound hydrogen atoms act as o-donors towards the transition metal, thus constraining the Si atom in close proximity to the transition metal even without o-donor action from the latter to the Si atom. Besides, there are review articles on this topic, as mentioned in the introduction [36-40]. Hence, compounds such as 158 and 159 will not be discussed in detail here [327, 328]. Also, the same applies to compounds with Si transition metal bond which only achieve Si-hypercoordination by the presence of metal-bonded hydrogen atoms in closer proximity of the Si atom, while the rest of the Si coordination sphere is almost tetrahedral, thus less indicative of H Si donor action (e.g., 160) [329],... 

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