The formation of HCp ttt was indeed confirmed by 1 H NMR analysis of the volatiles of the reaction (see details in the Supporting Information and Figure ). The thulium metal centers are hold together via two isomeric bridging {µ3-C7H7(C4O6)} 2-fragments. Fragment A (see Scheme 4) is best described by a µ3- ), the latter prepared using the same procedure but in toluene-d8 instead of protio-toluene. The IR spectrum of 9 shows a very intense absorption band at 1638 cm -1 , consistent with a coordinated ketone group, along with strong absorption bands at 1585 and 1552 cm -1 , which can be assigned to the carboxylate groups and/or delocalized C=C bond in the newly formed ligand. ), O12-C15 1.300( ), C1-C2 1.565( ), C2-C3 1.550( ), C2-C4 1.567( ), C13-C14 1.426(6), C14-C15 1.384(6).