Vibrational Solvatochromism in Vaska's Complex Adducts

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.