The major products of the reaction of laser ablated and excited U atoms and N-2 are the linear N U N dinitride molecule, isoelectronic with the uranyl dication, and the diatomic nitride U N. These molecules form novel cyclic dimers, (UN)(2) and (NUN)(2), with complex electronic structures, in matrix isolation experiments, which increase on UV photolysis. In addition, (NUN)(2) increases at the expense of (UN)(2) upon warming the codeposited matrix samples into the 20-40 K range as attested by additional nitrogen and argon matrix infrared spectra recorded after cooling the samples back to 4 or 7 K. These molecules are identified through matrix infrared spectra with nitrogen isotopic substitution and by comparing the observed matrix frequencies with those from electronic structure calculations. The dimerization is strong (theory predicts the dimer to be on the order of 100 kcal/mol more stable than the monomers), since the ground state involves 12 bonding electrons, 8 in the sigma-system, and 4 in the delocalized pi-system. This delocalized pi bonding is present in the U, Th, La, and Hf analogues further demonstrating the interesting interplay between the 5f and 6d orbitals in actinide chemistry. The (UN)(2)(+) cation is also observed in solid argon, and calculations indicate that the bonding in the ring is preserved. On the other hand, the NUN dimer is of lower C-2h symmetry, and the initial NUN molecules are recognizable in this more weakly bonded (Delta E = -64 kcal/mol) structure. The NThN molecules bind more strongly in the (NThN)(2) dimer than the NUN molecules in (NUN)(2) since NUN itself is more stable than NThN.