Gradient-corrected density functional calculations were used to determine the most stable geometrical conformations and lowest electronic states of molybdenum and niobium atoms, dimers, trimers, and tetramers and their adducts with dinitrogen. The binding energies, geometries, and vibrational frequencies of the most stable conformations and electronic states are compared to experimental and theoretical results to the extent they are available. To interpret some of the peculiar features of related experimental data, we studied the ionization potential and electronic properties of MoN and NbN and the binding of more than one nitrogen molecule to metal trimers.