Agostic interactions binding

Agostic interactions involving phosphines are quite significant because they possess residual binding sites for other small molecules such as H2, and phosphine groups play crucial roles in many homogeneous catalysts. Figure 11.5.8(a)... 

There is 3H and 13C NMR evidence for agostic interactions in Cd(II) and Hg(II) complexes of m-benziporphyrin.591 113Cd NMR spectroscopy was used to probe complexation of Cd2+ by 15-crown-5 and dibenzo-15-crown-5.592The binding of Cd(II) to 4-thiouridine or 4-thio-2 -deoxythymidine was follwed by 3H and 113Cd NMR spectroscopy.593... 

The precursor for the dihydrogen complex here contains a so-called agostic interaction [26], that is an intramolecular interaction of a C-H bond with the metal that stabilizes the electronically (16-electron) and coordinatively unsaturated W(CO)3(PTr3)2 species. It is noteworthy that the splitting/formation of the H-H bond is much slower here than its binding to and dissociation from the metal. 

Due to this lack of acidity, the internal carbon often does not activate upon metalation with many transition metals. This results in a novel form of porphyrin metal binding where the internal C-H bond(s) remain intact but lie close to the bound metal. Several metals exhibit this type of bonding, including manganese (25-27), rhodium (25), iron (29), cobalt (30 zinc (57), palladiiun (52), silver (55), cadmium, and mercury (34), The C-H bond can reside very close to the metal ion, often within 2.5 A. As a result of this proximity, this interaction has been referred to as agostic, however, the key measurement that would define the agostic interaction, the C- H coupling constant, has yet to be measured. 

Some C-H bonds of transition metal ligands are found to have an agostic interaction of type 12, where a C-H bond is oriented in a manner to intramolecularly bind to the metal. 

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