Relations between H-C and M-C bond strengths: influence of the organic ligand and the metal

This review describes the thermodynamics of C-H bond activation in various hydrocarbons, especially (poly)fluorinated benzenes, by transition metal complexes, through experiments and computational studies, with a focus on the latter. Experimental metal-carbon bond dissociation enthalpies (BDEs), determined relative to an internal reference, were used to establish the relationship between H-C and M-C BDEs. In parallel, bond dissociation enthalpies or energies were computed with DFT methods. For a given metal complex and a set of sufficiently similar organic substrates a linear correlation with a characteristic slope R M-C/H-C was obtained. Both experimental and computational studies indicate that the slope is always greater than 1 and quantitatively agree on its value when comparison could be made, suggesting a systematic thermodynamic preference for cleaving stronger hydrogen-carbon bonds. The R M-C/H-C value is particularly high for (poly)fluorobenzenes, with the greatest increase in M-C BDE occurring when fluorine atoms are ortho to the M-C bond, as found by experiments. The polarity