Zwitterionic metal alkyl species

Breslow suggested a mechanistic scheme (Scheme 8.1) involving a zwitterionic metal alkyl species, with a tetrahedral geometry of the titanium center. However, it was not possible at the time to provide direct structural evidence for these species. Alternative suggestions, e.g. by Olive et al. [11], included octahedral intermediates (Scheme 8.2), while an early proposal by Shilov et al. [12], based on electrochemical studies in dichloromethane, that the active species was likely to be... 

The species shown in Schemes 8.1 and 8.2 do not contain vacant coordination sites suitable for binding weakly donating ligands such as alkenes. Even in Breslow s zwitterionic intermediate (Scheme 8.1) the nature of the metal-ethene interaction is unclear alkenes do not bind to the LUMO of 16-electron complexes CP2MCI2 (M = Ti, Zr, Hf) or their alkyl derivatives. The isolation by Eisch in 1985 of a cationic titanium vinyl complex [Cp2TiC(Ph)=C(Me)SiMe3], apparently formed by insertion of an alkyne into a putative [Cp2TiMe] intermediate [29], raised the... 

The thermodynamic parameters for the abstraction process in Equation 12.7 have been measured by Marks. The transition metal products of these reactions are zwitterionic species contairung a bridging alkyl group. Representative enthalpies for this abstraction... 

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