An analysis of the geometries and electronic structures of a series of [LnM(CX3)] species (where X = H, F) is presented, on the basis of density functional theory (DFT) and the natural bonding orbital (NBO) approach. Computed geometries show that LnM-CF3 bonds can be up to 0.1 angstrom shorter than the equivalent LnM-CH3 bonds, although the extent of this shortening varies considerably depending on the LnM fragment. Evidence for CF3 having a higher trans influence than CH3 is seen, but this is most apparent in systems where the LnM-CF3 bond is itself shorter. NBO calculations show that the computed charge at the metal center is usually slightly more negative (or less positive) in the [LnM(CF3)] species compared to that of its CH3 congener. Further detailed NBO analyses on the [(H3P)(3)Rh(CX3)] (1), trans-[(H3P)(2)Pt(Cl)(CX3)] (2), [(OC)(5)Mn(CX3)] (3), and [Pt(H)(3)(CX3)](2-) (8) pairs indicate a significantly higher M <- CX3 sigma interaction when X = F. The LnM-CF3 sigma bond is computed to have much higher C 2s character and is also enhanced by contributions from the C-F sigma* orbitals. In contrast, any M -> C-F(sigma*) ir back-donation is relatively weak, being at most 8% of the magnitude of M <- CF3 sigma interaction, while M -> C-H(sigma*) pi back-donation is negligible in the [LnM(CH3)] congeners. The metal-based d orbitals are computed to be between 0.4 and 0.7 eV lower in energy in the [LnM(CF3)] species. Thus, CH3/CF3 replacement has two significant, apparently counterdirecting, effects, in that it both maintains and indeed can increase the electron density at the metal center, while at the same time causing a stabilization of the metal-based d orbitals. These effects account for the enhanced reactivity of [LnM(CF3)] species toward nucleophiles and form a basis for understanding the reactivity of [LnM(CF3)] species in the literature. Implications for the Pd-catalyzed trifluoromethylation of aryl halides ArX are discussed: in particular, the balance between Ar-CF3 reductive elimination from [LnPd(Ar)(CF3)] and the propensity of this species to undergo transmetalation (and hence catalyst deactivation) in the presence of [LnPd(Ar)(X)] species.
