Vibrational Couplings in Hydridocarbonyl Complexes: A 2D-IR Perspective

Hydridocarbonyl complexes,
a class of industrially relevant catalysts, contain both the M–H
and M–CO moieties. Here, using two-dimensional infrared spectroscopy,
we examine the coupling of the typically weak M–H stretching
mode and the intense M­(CO) mode. By studying a series of Ir­(I)-
and Ir­(III)-based hydridocarbonyl complexes, we show that the arrangement
of the H and CO ligands in a trans configuration
leads to strong vibrational coupling and mode delocalization. In contrast,
a cis arrangement leads to no coupling, with the
localized M–H mode having a much larger anharmonicity. These
results highlight a promising strategy for enhancing the M–H
vibration by intensity borrowing from the strong CO modes
in a trans configuration, allowing for direct determination
by infrared spectroscopy of both the oxidation (by frequency shifts)
and the protonation state (via vibrational coupling) of the complex,
in mechanistic studies of proton-coupled electron transfer reactions.