The vibrational solvatochromism of bis(triphenylphosphine) iridium(l) carbonyl chloride (Vaska's complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (upsilon(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 upsilon(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 upsilon(CO), supporting a model in which this ligand indirectly affects the carbonyl frequency by modulating the extent of metal-to-CO back-bonding. The upsilon(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 pi*-orbital.