Vibrational Solvatochromism in Vaska's Complex Adducts

Huber, C. J.; Anglin, T. C.; Jones, B. H.; Muthu, N.; Cramer, C. J.; Massari, A. M.
J. Phys. Chem. A 2012, 116, 9279 (doi:10.1021/jp3070536).

The vibrational solvatochromism of bis(triphenylphosphine) iridium(I) carbonyl chloride (Vaska's complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (ν_CO) was measured in 16 different organic solvents with a wide range of Lewis acidities for VC and its dioxygen (VC-O₂), hydride (VC-H₂), iodide (VC-I₂), bromide (VC-Br₂), and sulfide (VC-S_x) adducts. The ν_CO of the VC-O₂ complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts. The stretching frequency of the trans O₂ ligand on VC-O₂ was measured to be anticorrelated with ν_CO, supporting a model in which this ligand indirectly affects the carbonyl frequency by modulating the extent of metal-to-CO backbonding. The ν_CO values obtained from DFT calculations on VC adducts with solvent continua and explicit hydrogen bonds were used to aid the interpretations of the experimental results. The O₂ ligand is more susceptible to stronger specific solvent interactions and it binds in a fundamentally different mode from the monatomic ligands, providing a more direct communication channel with those metal d-orbitals that have the appropriate symmetry to backbond into the carbonyl π* orbital.