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 νCO ≈ 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 νCO ≈ 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.