Full Conformational Analyses of the Ultrafast Isomerization in Penta-coordinated Ru(S₂C₂(CF₃)₂)(CO)(PPh₃)₂: One Compound, Two Crystal Structures, Three CO Frequencies, 24 Stereoisomers, and 48 Transition States

The stereodynamics of an ultrafast (picosecond) isomerization in a penta-coordinated ruthenium complex, Ru­(S₂C₂(CF₃)₂)­(CO)­(PPh₃)₂, were characterized by density functional theory (DFT). The ruthenium complex crystallizes in two almost-square pyramidal (SP) forms. The violet form has an apical PPh₃ ligand, the orange form has an apical CO ligand, and their solution displays three CO stretching frequencies. With 4 possible centers of chirality (1 ruthenium, 2 phosphines, and 1 dithiolate), there are 24 stereoisomers. DFT calculations of these stereoisomers show structures ranging from almost-perfect SP (τ₅ ≈ 0) to structures significantly distorted toward trigonal bipyramidal (TBP) (τ₅ ≈ 0.6). The stereoisomers fall neatly into three groups, with ν_CO ≈ 1960 cm^–1, 1940 cm^–1, and 1980 cm^–1. These isomers were found to interconvert over relatively small barriers via Ru–S bond twisting, CF₃ rotation, phenyl twisting, PPh₃ rotation, and, in some cases, by coupled motions. The composite energy surface for each CO frequency group shows that interconversions among the low-energy structures are possible via both the direct and indirect pathways, while the indirect pathway via isomers in the ν_CO ≈ 1980 cm^–1 group is more favorable, which is a result consistent with recent experimental work. This work provides the first complete mechanistic picture of the ultrafast isomerization of penta-coordinated, distorted SP, d⁶-transition-metal complexes.