Understanding Terminal versus Bridging End-on N2 Coordination in Transition Metal Complexes

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 MN2 complexes into a bridging MNNM complex and a free N-2 molecule (2 Delta G(eq)(degrees)) is examined through systematic variations of the metal and ligands; values of Delta G(eq)(degrees) 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 pi-bond order (BO pi) to the triatomic terminal MNN moiety that is equal to that of the free N-2 molecule, and a variable BO pi to the bridging complexes based on the character (bonding or antibonding) and occupancy of the pi-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 pi-bonds (Delta BOeq pi > 0), the bridging mode is greatly favored; this condition is satisfied when each metal provides 1, 2, or 3 electrons to the pi-MOs of the MNNM core. When each metal in the bridging complex provides 4 electrons to the MNNM pi-MOs, Delta BOeq pi = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.

Automated summary

The study investigates the coordination modes of nitrogen to transition metal complexes and proposes a model explaining the coupling between terminal and bridging coordination. The conversion from terminal to bridging coordination is favored when it is associated with an increase in the number of pi-bonds, and the equilibrium can be shifted by dispersion interactions.