Oxygen-metal -bonding

Alkyllithium compounds as well as polymer-lithium associate not only with themselves but also with other alkalimetal alkyls and alkoxides. In a polymerization initiated with combinations of alkyllithiums and alkalimetal alkoxides, dynamic tautomeric equilibria between carbon-metal bonds and oxygen-metal bonds exist and lead to propagation centers having the characteristics of both metals, usually somewhere in between. This way, one can prepare copolymers of various randomness and various vinyl unsaturation. This reaction is quite general as one can also use sodium, rubidium or cesium compounds to get different effects. 

Numerous mechanistic investigations have been carried out (see an earlier summary2 and new publications, e.g. References 325, 337-339). The reaction of 38 or 39 is regarded as the RDS (Scheme 9). Of the two half-hydrogenated states (39 and 40), 39, in principle, is the more stable, since the oxygen-metal bond is stronger than the carbon-metal bond. 

Fig. 4.41. Molecules like CO are bonded in most of the group 8-10 metal surfaces with the molecular axis perpendicular to the surface (a) or slightly tilted. The amplitude of vibration around the surface normal increases with increasing temperature (b). At a certain temperature an oxygen-metal bond is formed (c) and dissociation occurs (d).

Both physical measurements and kinetic studies show that association between metal cations and enolate anions is stronger, i.e. the oxygen-metal bond has more covalent character, when small metal cations and less electropositive metals are involved. The degree of aggregation and extent of association of... 

This trend for subsurface accommodation of oxygen atoms at the late 4d TMs can be understood by looking at the subsurface geometries. In all relaxed structures, the subsurface oxygen-metal bond lengths are slightly larger than... 

This propagation reaction can be visualized as involving the formation of an incipient alkoxide anion on cleavage of the oxygen-metal bond in the propagating chain — hence the name anionic coordination. Many anionic coordination polymerizations proceed with stereochemical consequences. 

Although zinc ketone enolates are generally considered to exist with an oxygen-bound rather than a carbon-bound metal, there is still some controversy. Boersma assumes that they contain both zinc-carbon and zinc-oxygen bonds (Scheme 58). One indication that they contain only an oxygen-metal bond in solution is that the NMR data for zinc enolates are similar to the data for alkali metal enolates. ... 

In a monomolecular decomposition the direction of the process (the selectivity) must be determined from the structure of the transition state. It is acceptable that on those surfaces which show an affinity for hydrogen atoms dehydrogenation will be favored, and proceed via an intermediate in which the hydrogen of the formate ion has already formed a bond with a metal atom. In those cases where the oxygen-metal bond is much stronger than the Me-H-bond an OH group will be formed, which leads to dehydration. 

The activation energy for adsorption of oxygen on silver and platinum varies with surface coverage, which is indication that the metal surfaces are heterogeneous and the oxygen-metal bonds have different energies. 

Figure 44 Methyl acetate conversion at 197 °C over CufSi02 catalyst doped with different metal oxides (left-hand axis) and ethane selectivity (right-hand axis) at 247 °C as a function of oxygen-metal bond strength of the promoter (normalized per...

The nature of oxygen-metal bond affects structural properties. 

Formation of metal-oxygen-metal bond depends on donor-acceptor... 

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