Agostic Systems Containing Carbon-Hydrogen-Metal 3c-2e Bonds

AGOSTIC SYSTEMS CONTAINING CARBON-HYDROGEN-METAL 3c-2e BONDS 

Compound 89 is a six-coordinated 16-electron complex and the parent EtTiCh is a four-coordinated 12-electron complex. It is intriguing, therefore, that compound 89 has an agostic interaction, whereas EtTiCh, although more electron deficient, does not show any agostic behavior. Density functional theory (DFT) studies on these and other systems have been used to probe the conditions for the development of agostic interactions.  

Spectroscopic evidence is steadily accumulating that transient intermediates in reactions of hydrocarbons at metal centers involve what are referred to as a complexes, in which the pair of electrons from a C-H bond in the hydrocarbon are shared with a coordinatively unsaturated metal center to form a 3c-2e M—C—H bond, the ligand atoms of which subsequently separate on forming new bonds to other atoms in the coordination sphere of the metal concerned. Equation (2.11) is a general reaction to illustrate such transformations called a-CAM (a-complex-assisted metathesis) reactions with the involvement of the a complex 94. 

The cations 97 and 98 are examples of dinuclear metal complexes in which a methyl group attached by a normal metal-carbon bond to one metal atom leans over toward the second metal atom to allow C—H—Fe bonding to occur. This ensures that each iron atom in each complex has the full complement of 18 electrons required for coordinative saturation. A similar phenomenon is observed in the dicationic mixed Rh-Os complex 99. The methyl group in this complex, involved in the agostic bridged interaction, is surprisingly symmetrical. In fact, the Rh-C distance (2.29 A) is slightly shorter than the Os-C distance (2.32 A). 

Structural information for crystalline 100, particularly the acute W-C-H angle compared with that of W-C-H acquired by neutron diffraction, indicates one strong and one weak a-agostic interaction. However, the solution spectra ( H and C NMR) indicate fluxionaUty. 

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Figure 1.6. Agostic systems containing carbon-hydrogen-metal 3c-2e bonds ( denotes hypercarbons).

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