Understanding Terminal versus Bridging End-on N_2 Coordination in Transition Metal Complexes

摘要

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 (2△G_(eq)~o) is examined through systematic variations of the metal and ligands; values of △G_(eq)~o 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 (△BO_(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, △BO_(eq)~π = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.