The 1 H NMR spectrum of 3 at room temperature revealed several broad resonances in the range d -100 to 400 ppm, as a result of the paramagnetic nature of the complex and restricted free rotation of the Cp ttt ligands. Upon heating to 80 °C (see Figure ), a simpler baseline was observed featuring four main and relatively broad (Dn1/2 ≈ 1500-3200 Hz) resonances at d 185, 153, 32 and -33 ppm, assigned to the t Bu groups. The number of signals and the corresponding integrations support free rotation of the Cp ttt ligands at high temperature, resulting in an overall Cs symmetry in solution. It should be noted that complex 3 was found to be thermally stable at least up to 110 °C in toluene solution. As a result of the highly paramagnetic nature of the complex, no signals could be observed in the 13 C{ 1 H} NMR spectra of both 3 and 3-13 C. In the IR spectrum of 3, a very strong absorption band was observed at 2066 cm -1 in the ketene region, shifting to 2003 cm -1 in 3-13 C upon 13 CO isotope labelling (See Figure ). In order to confidently assign the corresponding absorption band, the vibrational spectra of 3 and 3-13 C were calculated at the DFT level of theory (B3PW91, see details in the Supporting Information). In the computed spectra (Figures ), the asymmetric stretch of the O1-C1-C2 ketene fragment gives rise to an intense absorption band at 2190 cm -1 in 3 and 2121 cm -1 in 3-13 C. Although the (uncorrected) computed values are ca. 120 cm -1 larger than the experimental ones, the ratios between the wavenumbers of two isotopologues (1.031 experimentally and 1.033 computationally) are almost identical, confirming the assignment of the band.