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

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Nitrogen (N2) coordination to transition metals can occur in terminal or bridging modes.
• These coordination modes are critical for understanding N2 activation in processes like ammonia synthesis.
• Predicting the favored binding mode for specific metal-ligand systems remains a challenge.

Purpose of the Study:

• To elucidate the fundamental factors governing terminal versus bridging N2 coordination to transition metal complexes.
• To determine which binding mode is favored for a given metal-ligand system.
• To provide insights for designing improved catalysts for N2 functionalization and ammonia synthesis.

Main Methods:

• Quantitative density functional theory (DFT) calculations were employed.
• Qualitative molecular orbital (MO) analyses were performed.
• Systematic variations of metals and ligands were examined to study the Gibbs free energy of N2 coordination.

Main Results:

• The Gibbs free energy for converting terminal to bridging N2 complexes (2ΔGeq°) varies broadly (-24.0 to +9.1 kcal/mol per M-N2 bond).
• A model is proposed based on π-bond order (BOπ) changes during the terminal-to-bridging conversion.
• Bridging coordination is favored when the conversion increases the total π-bond order (ΔBOeqπ > 0), typically when metals contribute 1-3 electrons to the MNNM core π-MOs.

Conclusions:

• The favored N2 binding mode (terminal vs. bridging) is predictable based on the electronic contribution of the metal to the MNNM π-system.
• When metals contribute 4 electrons to the MNNM π-MOs, the equilibrium is near-ergoneutral, influenced by dispersion forces.
• This work provides a fundamental understanding of N2 coordination, crucial for designing catalysts for nitrogen fixation.