Olefin transition metal complexes length

Powder as well as MAS studies are available for several r/ -bonded olefins. The spans of the chemical-shift tensors are reduced with respect to the free olefins, which is discussed in terms of the Dewar-Chatt-Duncanson model of fx-donation and tt- back-bonding." " " The bond lengths and the orientations of the shielding tensor elements are available from dipolar-chemical shift methods and 2-D spin-echo experiments on the doubly labelled oleflns." " 77 -cyclopentadienyl and 77 -benzene ligands of transition-metal complexes, but also some derivatives of alkali or main-group elements," exhibit single resonances and shielding tensors of axial symmetry at room temperature. Both observations point to relatively fast rotations around the respective 5- and 6-fold local rotor axis. ... 

In spite of the presence of usually bulky substituents at the silicon atoms, the Si—Si bond lengths are dramatically shortened and approach those of disilenes. This bond shortening is accompanied by a planar or almost planar arrangement of the substituents R1, R2 and the other silicon atom about each silicon atom. Such a geometrical arrangement resembles the bonding situation in transition metal-olefin complexes which, according to the model... 

Like strained olefins, strained alkynes, some of which are unstable when free in solution, form stable complexes with transition metals. Cycloheptyne, cyclohexyne, cyclopentyne, and benzyne complexes have aU been prepared. The lengths of the coordinated and uncoordinated C-C bonds of most benz)me complexes are equivalent. Thus, the structures and reactions of the benzyne complexes are more like those of arene dianions than those of a fragment containing a strained C-C triple bond. In other words, these complexes are better described by resonance structure B in Figure 2.27 than by resonance structure A. 

If the IT backbonding becomes even more important, the lengthening of the C-C bond will be such that this bond will reach the length of a single bond. In this extreme case, it is appropriate to change its representation. The system should then no longer be viewed as a metal-olefin complex, but as a metallacyclopropane. This reasoning can be extended to metal-alkyne complexes and to the bond between the carbonyl of ketones and very oxophilic early transition-metals such as Sc and Ti. 

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