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-O 2), hydride (VC-H 2), iodide (VC-I 2), bromide (VC-Br 2), and sulfide (VC-S X) adducts. The ν CO of the VC-O 2 complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts. The stretching frequency of the trans-O 2 ligand on VC-O 2 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 back-bonding. 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 O 2 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 back-bond into the carbonyl π*-orbital.