(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 N2. 36b Crystals of 5 suitable of X-ray diffraction studies revealed the formation of a centrosymmetric dinuclear Lu III complex featuring an end-on bridging (N2) 2-ligand (Figure ). Although N2 coordination on low-valent RE complexes is relatively common, the end-on -    -N2 coordination mode is still rare, 36b,53 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. 36b The N-N separation of 1.203(11) Å is consistent with a doubly reduced (N2) 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 N2 stretch (Figure ). This assignment was confirmed by the shift of this signal to 1559 cm -1 for the 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 N2 activation in rare-earth metal diazenido complexes seems to particularly depend on the (N2) 2-coordination mode. For the most common side-on coordination mode, the N2 stretch lies in the range 1371-1473 cm -1 , 11c,54 whereas the reported N2 stretching vibration in end-on complexes is at higher energy, ranging from 1595 to 1660 cm -1 . 36b,53 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(-    -N2)] (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 N2 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 (N2) 2-ligand can adopt a triplet ground state by occupying the two formally degenerated * orbital of N2. 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 1.8 kcal•mol -1 , 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 N2 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 dxy orbitals overlap with one of the N2 * while the second * orbital overlaps with the dxz in the LUMO (Figure ).