Allene Complexes Bonding and Structure

Structures of these compounds are analogous to those of olefin and alkyne complexes. In square-planar compounds, the allene molecule is approximately perpendicular to the ML3 plane, while in the remaining types of compounds, the allene molecule lies almost in the ML2 plane. The angle between this plane and the plane containing the allene carbon atoms equals about 10°. The character of the metal-allene bond may be described by means of the Dewar-Chatt-Duncanson model as in the case of alkene and acetylene complexes. The formation of the a(d -n) and n d -n ) bonds causes an increase in the carbon-carbon distance and tilting of the allene substituents away from 

The distance a between the middle carbon and the metal is shorter than the distance b between the central atom and the terminal allene carbon atom. 

Kashiwagi, M. Kukudo, and N. Yasuoka, Chem. Commun., 317 (1969). 

Like in the case of olefin complexes, the chemical shift t of the coordinated CHR group increases by 2-3 ppm compared to the free allene. This is also in agreement with the Dewar-Chatt-Duncanson model of the metal-allene bond (rehybridization sp - sp ) according to the valence bond theory. 

Three signals are observed in the NMR spectrum. The highest t values are seen for a and b protons, while the lowest value is for c see structure (6.211). The coupling 

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