Terminal and bridging end-on coordination of N 2 to transition metal complexes offer possibilities for distinct pathways in ammonia synthesis and N 2 functionalization. Here we elucidate the fundamental factors controlling the two binding modes and determining which is favored for a given metal−ligand system, using both quantitative density functional theory (DFT) and qualitative molecular orbital (MO) analyses. The Gibbs free energy for converting two terminal MN 2 complexes into a bridging MNNM complex and a free N 2 molecule (2ΔG eq °) is examined through systematic variations of the metal and ligands; values of ΔG eq °range between +9.1 and −24.0 kcal/mol per M−N 2 bond. We propose a model that accounts for these broad variations by assigning a fixed π-bond order (BO π ) to the triatomic terminal MNN moiety that is equal to that of the free N 2 molecule, and a variable BO π to the bridging complexes based on the character (bonding or antibonding) and occupancy of the π-MOs in the tetratomic MNNM core. When the conversion from terminal to bridging coordination and free N 2 is associated with an increase in the number of π-bonds (ΔBO eq π > 0), the bridging mode is greatly favored; this condition is satisfied when each metal provides 1, 2, or 3 electrons to the π-MOs of the MNNM core. When each metal in the bridging complex provides 4 electrons to the MNNM π-MOs, ΔBO eq π = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.