Combined triple and double bonds to uranium: The N≡U=N-H uranimine nitride molecule prepared in solid argon

Reactions of laser-ablated U atoms with N₂ and H₂ mixtures upon codeposition in excess argon at 5 K gave strong NUN and weak UN infrared absorptions and new bands at 3349.7, 966.9, 752.4, and 433.0 cm-1 for the unusual new U(V) molecule N≡U=N-H, uranimine nitride, containing both triple and double bonds. This identification is based on D and 15N isotopic substitution and comparison with frequencies computed by density functional theory for the 2Δ ground state NUNH. Calculated bond lengths are compared to those of the 1∑_g+ ground state of U(VI) uranium dinitride N≡U=N, the 2Φ ground state of the isoelectronic nitride oxide N≡UdO, and the 3A ground state of the U(IV) uranimine dihydrideN≡U= N-H molecule, which have all been prepared in solid argon matrices. Mulliken bond orders based on the CASSCF orbitals for N≡U=N-H are 2.91, 2.19, and 1.05, respectively. Here, the terminal nitride is effectively a triple bond, just as found for N≡U=N. The solid argon matrix is a convenient medium to isolate reactive terminal uranium nitrides for examination of their spectroscopic properties.