Laser-ablated yttrium and lanthanum hydrides have been codeposited at 4 K with dinitrogen in excess argon. Products, HY(N-2), HYNN, H2YNN, HLaNN, and H2LaNN, have been formed in the present experiments and characterized by using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, and comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed for the formation of these molecules.