Ultrafast dynamics in iron tetracarbonyl olefin complexes investigated with two-dimensional vibrational spectroscopyElectronic supplementary information (ESI) available: Delay scans for delay times up to 150 ps and a least-squares fit to determine T1; singular-value decomposition of the delay-dependent Fe(CO)4(cinnamic acid) and Fe(CO)4(dimethyl fumarate) data; singular value decomposition of the delay-dependent intensity of the cross peaks of the Fe(CO)4(cinnamic acid) and Fe(CO)4(dimethyl fuma

The dynamics of iron tetracarbonyl olefin complexes has been investigated using two-dimensional infrared (2D-IR) spectroscopy. Cross peaks between all CO-stretching bands show that the CO-stretch modes are coupled, and from the cross-peak anisotropies we can confirm previous assignments of the absorption bands. From the pump-probe delay dependence of the diagonal peaks in the 2D-IR spectrum we obtain a correlation time of ∼3 ps for the spectral fluctuations of the CO-stretch modes. We observe a multi-exponential pump-probe delay dependence of the cross-peak intensities, with rate constants ranging from 0.1 ps −1 to 0.6 ps −1 . To determine whether this delay dependence originates from fluxionality of the complex or from intramolecular vibrational relaxation (IVR), we modulate the free-energy barrier of fluxional rearrangement by varying the pi-backbonding capacities of the olefin ligand in two iron tetracarbonyl olefin complexes: Fe(CO) 4 (cinnamic acid) and Fe(CO) 4 (dimethyl fumarate). Since the pi-backbonding strongly influences the rate of fluxionality, comparing the dynamics in the two complexes allows us to determine to what extent the observed dynamics is caused by fluxionality. We conclude that on the time scale of our experiments (up to 100 ps) the cross-peak dynamics in the iron complexes is determined by intramolecular vibrational energy relaxation. Hence, in contrast to previously investigated irontricarbonyl and ironpentacarbonyl complexes, iron tetracarbonyl olefin complexes exhibit no fluxionality on the picosecond time scale. We unravel the dynamics of CO-stretch excitations in iron tetracarbonyl olefin complexes using 2D-IR spectroscopy and by modulating the energy barrier for fluxional rearrangement.