Intermediates agostic bond

In the context of 1H chemical shifts and determination of the reaction mechanism of homogeneous hydrogenation catalysts, one usually tries to observe hydride-intermediates that typically resonate at high field (-5 to -30 ppm). Agostic bonds (see Fig. 11.1) also tend to have a hydride-like proton chemical shift. 

The agostic interaction is considered to be important in such reactions as a elimination, /1-hydrogen elimination, and orthometalation. For example, in a/i-hydrogen elimination, a H atom on Lhc/i-C atom of an alkyl group is transferred to the metal atom and an alkene is eliminated. This reaction proceeds through an agostic-bond intermediate, as shown below ... 

This decomposition pathway leads to equimolar amounts of alkenes and alkanes. Complexes with /3-agostically bonded alkyl groups are likely intermediates structures of this kind may sometimes be the ground state, as in [(P—P)Pt(r72-C2H5)]+., 5... 

While in manganese and other electronically saturated complexes an -bonded acyl (21-XXIXb) is observed as an unstable intermediate, there are numerous examples of stable V-acyls of Group 4-6 metals.153 For molybdenum a third, j8-agostic bonding mode was observed which is in equilibrium with the alkyl-carbonyl isomer 154... 

Thus, a system with a C-H — M agostic bond is, as it were, on the reaction pathway between the C-H -i- M system and the alkyl hydride derivative C-M-H. In recent years, compounds containing agostic bonds have been proposed as intermediates in reactions that involve C-H bond activation. For example, in the thermolytic rearrangement of cw-bis(silylmethyl)platinum(II) complexes, a mechanism is proposed which involves preliminary dissociation of one Pt-P bond compensated by an agostic interaction between tbe coordinatively unsaturated metal and a phosphine substituent (VI.2) [8]. 

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. 

The insertion reaction, as its reverse, involves a metal-olefin-hydride species, and thus requires, in one direction or the other, a vacant site. It is thus necessary to start from a complex that has at most 16 valence electrons. The transition state then involves a square structure, then the last intermediate has a M-H-C agostic bond in equilibrium with the final complex. 

The polymerization is not interrupted when the olefin is in excess the initiator with d complexes, because extrusion is disfavored by the instability of the intermediate olefin complex due to the absence of it backbonding. This is not the case for the metals located on the right of the periodic table, rich in electrons, for which the insertion of an olefin onto a metal-alkyl bond is less frequent and extrusion very favorable in the presence of a vacant coordination site. If even a late metal is sufficiently electron poor, however, as for instance in cationic complexes, extrusion can be unfavorable and insertion leads to rapid polymerization, rookhart has shown that the key to this polymerization with o p OMe 3 H5 is in the agostic p -H bond with the metal, a witness of the low metal basicity. The coordination site that is occupied by the agostic bond must become vacant in order to let the olefin enter the coordination sphere... 

---