Reactivity of Tantalum Carbide Cluster Anions TaC n- ( n = 1-4) with Dinitrogen.

Dinitrogen activation/fixation is one of the most important and challenging subjects in synthetic as well as theoretical chemistry. In this study, the adsorption reactions of N2 onto TaC n- ( n = 1-4) cluster anions have been investigated by means of mass spectrometry in conjunction with density functional theory calculations. Following the experimental results that only TaC4- was observed to adsorb N2, theoretical calculations predicted that the TaC4- reaction system (TaC4- + N2 → TaC4N2-) has a negligible barrier on the approach of N2 molecule while insurmountable barriers are located on the reaction pathways of TaC1-3-/N2 reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC1-4- would facilitate the initial approach of the nonpolar N2 molecule, and the appropriate frontier orbital of TaC1-4- with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N2 molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N2 adsorption.