(5) (Figure ). This work parallels the recent synthesis by Layfield and co-workers of the analogous Gd, Tb, and Dy complexes by in situ reaction of the elusive divalent complexes with N Crystals of 5 suitable of X-ray diffraction studies revealed the formation of a centrosymmetric dinuclear Lu III complex featuring an end-on bridging (N 2 ) 2-ligand (Figure ). Although N 2 coordination on low-valent RE complexes is relatively common, the end-on -    -N 2 coordination mode is still rare, 35b,50 typically favored by bulky ligands and RE metals of small ionic radii. As expected from the lanthanide contraction, the Lu-N separation of 2.184(5) Å is slightly shorter than the Ln-N separations of 2.325(4), 2.296(4) and 2.268(7) Å in the Gd, Tb and Dy analogues, respectively. The N-N separation of 1.203(11) Å is consistent with a doubly reduced (N 2 ) 2-ligand, which is corroborated by vibrational spectroscopy. Although the IR spectrum of 5 is very similar to that of 2 (Figure ) because the N=N vibration is not IR active in this centrosymmetric structure, the Raman spectrum of 5 revealed a sharp band at 1610 cm -1 for the symmetric N 2 stretch (Figure ). This assignment was confirmed by the shift of this signal to 1559 cm -1 for the 15 N-labeled complex 5-15 N. The experimental ratio of frequencies of 1.033 matches the theoretical value of 1.035 derived from reduced mass considerations. It is worth noting that the extent of N 2 activation in rare-earth metal diazenido complexes seems to particularly depend on the (N 2 ) 2-coordination mode. For the most common side-on coordination mode, the N 2 stretch lies in the range 1371-1473 cm -1 , 11c,51 whereas the reported N 2 stretching vibration in end-on complexes is at higher energy, ranging from 1595 to 1660 cm -1 . 35b,50 Once formed, 5 is only sparingly soluble in hydrocarbon solvents and readily decomposes in ether solvents to unidentified species, which hampered analysis by NMR spectroscopy. The UV-vis spectrum in pentane of in situ formed 5 shows the disappearance of the characteristic band of 2 centered at 435 nm, at the expense of two intense absorption bands at 310 ( = 5900 L•mol -1 •cm -1 ) and 555 nm ( = 5300 L•mol -1 •cm -1 ) (Figure ). The latter band is slightly blue-shifted compared to that in the corresponding Dy complex, following the preliminary trend observed in the [{Ln(Cp ttt ) 2 } 2 (-    -N 2 )] (Ln = Gd, Tb, Dy) series. The structure of complex 5 was investigated computationally using the same methodology as before. Both end-on and side-on coordination of N 2 were considered and the end-on coordination is found to be the most favorable. In the same way, the singlet and triplet spin states were considered for complex 5, as a recent report by Jones and co-workers demonstrated that the (N 2 ) 2-ligand can adopt a triplet ground state by occupying the two formally degenerated * orbital of N 2 . A similar triplet ground state was observed by Evans and co-workers for end-on coordinated dinitrogen Sc and Gd ionpair complexes supported by silylamide ligands. In the case of 5, the singlet is found to be slightly more stable than the triplet by , consistent with the recent findings of Layfield and coworkers on the Gd, Tb and Dy analogues. The singlet optimized geometry compares well with the experimental one (Lu-N distance of 2.20 Å and N-N distance of 1.20 Å), in line with a two-electron reduction of N 2 upon coordination to two Lu II fragments. This reduction, evidenced by the N-N distance variation, is also highlighted by the nature of the HOMO where two d xy orbitals overlap with one of the N 2 * while the second * orbital overlaps with the d xz in the LUMO (Figure ).