The isotopically labelled complex [Tm(Cp ttt )2]2(µ-k(O):k(Oʹ)-13 C2O2) (2-13 C) was obtained by the same procedure using 13 C-enriched 13 CO. As expected, the IR spectra of 2 and 2-13 C are nearly identical (see Figure ), one only notable difference being the band at 1329 cm -1 in 2 that is red-shifted to 1304 cm -1 upon 13 C labelling. The corresponding absorption band can be confidently assigned to the C-O stretching vibration, as the experimental ratio between these numbers, 1.019, compares well with the theoretical value of 1.023 obtained from reduced mass considerations. Owing to highly paramagnetic nature of Tm(III) complexes, no C NMR resonances could be detected, even for the isotopically enriched 2-13 C.