Organometallic Compounds with Perfluorinated Ligands

Transition metal complexes containing fluorinated ligands date back to the initial explosive development of organometallic chemistry in the period from 1950 to 1970, and some early reviews contain a great deal of useful and relevant information (3,6,7). Early work clearly established that these compounds do indeed exhibit dramatically different structural, bonding, and chemical 

The only quantitative numerical data that allow a comparison of the relative thermodynamic stabilities of M—R and M—Rf a bonds are the bond dissociation energies D([CF3—Mn(CO)5]) = 172 7 kJ/mol and D([CH3—Mn(CO)5]) = 153 5 kJ/mol (II). The value for the CF3 complex is identical within experimental error to that found for the corresponding Mn—C(sp2) bond in the phenyl derivative (11). A relative strengthening of the Mn—CF3 bond compared to the Mn—CH3 bond is implied by the fact that the difference (19 kJ/mol) between the values for the CF3 and CH3 bonds to manganese exceeds that (6 kJ/mol) between D(H—CF3) and D(H—CH3) (11). 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition 

The syntheses, structures, spectroscopy, bonding, and reactivity of difluoromethylidene and other halocarbene complexes have been reviewed recently (10). Only some typical structures and more recent results are discussed briefly here. 

In contrast, the plane of the CF2 ligand in the osmium complex 6 lies perpendicular to the equatorial plane of the trigonal bipyramid, the conformation expected in order to maximize back bonding, and the rate of rotation of the CF2 ligand is slow on the NMR time scale, due to more extensive back bonding from the Os(O) center (70). 

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