The 2,2,2-Trifluoroethyl Ligand: A Structural “Reporter” Group for Its Carbanion Character in Organometallic Compounds

The crystal and molecular structures of the complexes trans-bis(dimethylglyoximato)(trifluoroethyl)(L)cobalt(III) [L = 4-cyanopyridine (4-CNpy) (I), triphenylphosphine (PPh₃) (II), trimethyl phosphite (III)] are reported. Crystal data for I:P2₁/c, a = 14.03 (1) Å, b = 8.518 (7) A, c = 17.73 (1) Å, β = 113.0 (1)°, Z = 4, R = 0.032 for 2468 independent reflections. Crystal data for II: P2₁/n, a = 19.801 (1) Å, b = 13.295 (7) Å, c = 14.494 (8) Å, β = 131.0 (4)°, Z = 4, R = 0.029 for 2685 independent reflections. Crystal data for III: P1, a = 10.020 (8) Å, b = 15.65 (1) Å, c = 16.06 (1) Å, α = 86.3 (1)°, β = 76.0 (1)°, γ = 116.8 (1)°, Z = 4, R = 0.041 for 3915 independent reflections. In all cases, the CH₂CF₃ group has a geometry that departs significantly from that expected for sp³ hybridization at the C bearing the F substituents. This departure was such that the C-C-F angles were much larger and the F-C-F angles were much smaller than expected. The values of the experimental angles were compared with those obtained from MNDO calculations (using several different initial assumptions) on CH₃CF₃, CH₂CF₃, and CH₂CF₃^- species. A plot of the average F-C-F angle vs. the average C-C-F angle for each determination in both the theoretical and experimental studies conformed to the expected geometric relationship, and the position of the point for each determination appears to be indicative of the carbanion character of each compound. Compound II occurs at a position even more “carbanionic” in character than does the calculated carbanion. Compounds I and III are less carbanionic in geometry, but III occurs as two crystallographically independent species which differ somewhat in carbanion character, possibly due to lattice effects. One view of this deviation from sp³ geometry is that the hybridization favors p character in the C-F bonds and s character in the C-C bonds of the CH₂CF₃ groups. This view is supported by “hybridization indexes” calculated according to methods developed by others. It is suggested that the geometry of the -CH₂CF₃ group indirectly furnishes some clue as to the electron donation by the alkyl group to the metal. The group thus is a structural reporter group for carbanion character of the alkyl.