Table 4. Calculated Reaction Free Energies and Free Energy Barriers (kcal/mol) for Rotation from Tetrahedral to Square-Planar Geometry at the M(dppe) Site in Compounds 1–3.
| complex | ΔG° DFT | ΔG† DFTa | ΔG† exptc |
|---|---|---|---|
| (OC)3Fe(pdt)Ni(dppe) (1) | –0.67 | 6.66 | 9.5 |
| (OC)3Fe(pdt)Pd(dppe) (2) | +0.99 | 3.29 | <7.2 |
| (OC)3Fe(pdt)Pt(dppe) (3) | –30.42 | NDb | ND |
The potential energy surface was found to be extremely flat along the isomerization pathway. As a result, the single imaginary frequency for the transition state (TS) was <20 cm–1 for the two barriers given in this table. Each TS was verified to lead to the relevant tetrahedral and square-planar geometries by following the intrinsic reaction coordinate (IRC) for 4 or 5 steps in both directions and subsequently optimizing the geometries. As the imaginary frequencies may be below the numerical accuracy of the methodology, and the complete IRC was not obtained, the free energy barriers should be viewed with caution. Moreover, multiple TSs were found for 1, suggesting a ruffled potential energy surface connecting the two isomers, and only the highest free energy barrier is reported.
The TS was not determined for 3 as the isomerization was found to be significantly exergonic.
Estimated according to the Gutowsky–Holm relation using the coalescence temperature of 243 K (detailed in SI), but the DFT free energies were calculated at 298 K for consistency with other experiments. Note that the calculated and experimental free energy barriers are not exactly equivalent.