Natural Bond Orbital Analysis of the Electronic Structure of [LnM(CH₃)] and [LnM(CF₃)] Complexes

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 Å 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)3Rh(CX3)] (1), trans-[(H3P)2Pt(Cl)(CX3)] (2), [(OC)5Mn(CX3)] (3), and [Pt(H)3(CX3)]2– (8) pairs indicate a significantly higher M←CX3 σ interaction when X = F. The LnM–CF3 σ bond is computed to have much higher C 2s character and is also enhanced by contributions from the C–F σ* orbitals. In contrast, any M→C–F(σ*) π back-donation is relatively weak, being at most 8% of the magnitude of M←CF3 σ interaction, while M→C–H(σ*) π 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.