Alkene 7T-complex

The question as to whether a transition metal complex of type 4 is best described as an alkene 7T-complex 4A or as a metallacyclopropane 4B, which is of practical importance, has been addressed in several computational studies on the relationship between alkene 7T-complexes and three-membered rings [48—52]. It has been concluded that the titanium complexes of type 4 are best represented as titanacydopropanes, i.e. by resonance structure 4B, if one is willing to accept the notion that 4A and 4B are limiting resonance forms [52],... 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

In the absence of a vinyl group or a second cyclopropane ring, bis(benzonitrile)palladium(II) chloride forms alkene 7t-complexes, e.g. 14, as the result of the cleavage of a cyclopropane bond and addition of chloride. ... 

A limited number of cyclometalation reactions at alkene and related C-H bonds have been carried out. In part, this may have arisen because such ligands can form Pd-alkene 7T-complexes rather than give C-H insertion products. Nevertheless, examples can be found, as in the cyclometalation of vinylic oximes (equation 74). 

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See also in sourсe #XX -- [ ]

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