How does the Environment Influence a Given Cation? A Systematic Investigation of [Co(CO) 5 ] + in Gas Phase, Solution, and Solid State.

IR spectroscopic studies of the gaseous metal carbonyl cations [Co(CO) ₅ ] ⁺ ⋅mCO (m=1-4) indicated that the weakly bound CO molecules in a second coordination sphere perturb the structure of [Co(CO) ₅ ] ⁺ causing the CO stretching frequencies ν(CO) to become noticeably redshifted. In this work, we aimed to establish the relationship between such gas phase IR spectra and those recorded in condensed phases, either as a solid salt or as a solution in the weakly basic solvent o-difluorobenzene. For this purpose, a series of [Co(CO) ₅ ] ⁺ [WCA] ^- salts (WCA=weakly coordinating anion), with the counterions varying between more coordinating (WCA=F-Al(OR ^F ) ₃ , (R ^F O) ₃ Al-F-Al(F)(OR ^F ) ₂ ; R ^F =C(CF ₃ ) ₃ ) and almost non-coordinating (WCA=Al(OR ^F ) ₄ , F{Al(OR ^F ) ₃ } ₂ ), were synthesized and characterized by vibrational spectroscopy as well as X-ray structure analysis. The experimental spectra differ considerably from that of the undisturbed gaseous [Co(CO) ₅ ] ⁺ ion, as the structural deformation of [Co(CO) ₅ ] ⁺ requires very little energy. Together with previously reported data, the perturbed condensed phase [Co(CO) ₅ ] ⁺ ions were analyzed and compared with the gaseous [Co(CO) ₅ ] ⁺ ⋅mCO ions. DFT calculations were performed on simply adapted [Co(CO) ₅ ] ⁺ structures to allow the assignment of all the ν(CO) modes and a qualitative interpretation of structural deformations by external influences as a function of the environment (ligands, solvation, crystal packing). The analysis showed that especially the degenerate E' mode ν _e and the averaged asymmetric equatorial CO stretch ν ‾ _e , which originates from a split of the E' mode, are a function of the interaction with the environment. Whereas for the more coordinating counterions ν ‾ _e values of 2112-2120 cm ^-1 were obtained, for the less coordinating counterions ν ‾ _e values of up to 2133 cm ^-1 were found, which is very close to that of gaseous [Co(CO) ₅ ] ⁺ ⋅4CO, with a ν ‾ _e value of 2135 cm ^-1 . This indicates the possibility of approximating the gas phase conditions in the condensed phases with the [Co(CO) ₅ ] ⁺ ion probably being the prototypical probe molecule for investigating the strengths of interactions in different environments.
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